diff --git a/IPython/html/static/base/js/utils.js b/IPython/html/static/base/js/utils.js
index 22261f3e4..16f9a7068 100644
--- a/IPython/html/static/base/js/utils.js
+++ b/IPython/html/static/base/js/utils.js
@@ -563,6 +563,22 @@ define([
         );
     };
     
+    /** Error type for wrapped XHR errors. */
+    var XHR_ERROR = 'XhrError';
+    
+    /**
+     * Wraps an AJAX error as an Error object.
+     */
+    var wrap_ajax_error = function (jqXHR, status, error) {
+        var wrapped_error = new Error(ajax_error_msg(jqXHR));
+        wrapped_error.name =  XHR_ERROR;
+        // provide xhr response
+        wrapped_error.xhr = jqXHR;
+        wrapped_error.xhr_status = status;
+        wrapped_error.xhr_error = error;
+        return wrapped_error;
+    }
+    
     var utils = {
         regex_split : regex_split,
         uuid : uuid,
@@ -588,6 +604,8 @@ define([
         ajax_error_msg : ajax_error_msg,
         log_ajax_error : log_ajax_error,
         requireCodeMirrorMode : requireCodeMirrorMode,
+        XHR_ERROR : XHR_ERROR,
+        wrap_ajax_error : wrap_ajax_error
     };
 
     // Backwards compatability.
diff --git a/IPython/html/static/notebook/js/menubar.js b/IPython/html/static/notebook/js/menubar.js
index 8e14bc2dc..01b09ae0e 100644
--- a/IPython/html/static/notebook/js/menubar.js
+++ b/IPython/html/static/notebook/js/menubar.js
@@ -91,23 +91,17 @@ define([
             // notebook's path.
             that.contents.new_notebook(that.notebook.notebook_path,
                 {
-                    success: function (data, status, xhr) {
+                    success: function (data) {
                         window.open(
                             utils.url_join_encode(
                                 that.base_url, 'notebooks',
                                 data.path, data.name
                             ), '_blank');
                         },
-                    error: function(xhr, status, error) {
-                        var msg;
-                        if (xhr.responseJSON && xhr.responseJSON.message) {
-                            msg = xhr.responseJSON.message;
-                        } else {
-                            msg = xhr.statusText;
-                        }
+                    error: function(error) {
                         dialog.modal({
                             title : 'Creating Notebook Failed',
-                            body : "The error was: " + msg,
+                            body : "The error was: " + error.message,
                             buttons : {'OK' : {'class' : 'btn-primary'}}
                         });
                     }
diff --git a/IPython/html/static/notebook/js/notebook.js b/IPython/html/static/notebook/js/notebook.js
index fdc1c615b..05d1c06aa 100644
--- a/IPython/html/static/notebook/js/notebook.js
+++ b/IPython/html/static/notebook/js/notebook.js
@@ -1923,7 +1923,7 @@ define([
         this.contents.save_file(this.notebook_path, this.notebook_name, model, {
                 extra_settings: extra_settings,
                 success: $.proxy(this.save_notebook_success, this, start),
-                error: function (xhr, status, error) {
+                error: function (error) {
                     that.events.trigger('notebook_save_failed.Notebook');
                 }
             });
@@ -1935,10 +1935,8 @@ define([
      * @method save_notebook_success
      * @param {Integer} start Time when the save request start
      * @param {Object} data JSON representation of a notebook
-     * @param {String} status Description of response status
-     * @param {jqXHR} xhr jQuery Ajax object
      */
-    Notebook.prototype.save_notebook_success = function (start, data, status, xhr) {
+    Notebook.prototype.save_notebook_success = function (start, data) {
         this.set_dirty(false);
         if (data.message) {
             // save succeeded, but validation failed.
@@ -2078,7 +2076,7 @@ define([
         var that = this;
         this.contents.rename_file(this.notebook_path, this.notebook_name,
                                   this.notebook_path, new_name, {
-            success: function (json, status, xhr) {
+            success: function (json) {
                 var name = that.notebook_name = json.name;
                 that.session.rename_notebook(name, json.path);
                 that.events.trigger('notebook_renamed.Notebook', json);
@@ -2091,12 +2089,12 @@ define([
         this.contents.delete_file(this.notebook_name, this.notebook_path);
     };
 
-    Notebook.prototype.rename_error = function (xhr, status, error) {
+    Notebook.prototype.rename_error = function (error) {
         var that = this;
         var dialog_body = $('<div/>').append(
             $("<p/>").text('This notebook name already exists.')
         );
-        this.events.trigger('notebook_rename_failed.Notebook', [xhr, status, error]);
+        this.events.trigger('notebook_rename_failed.Notebook', error);
         dialog.modal({
             notebook: this,
             keyboard_manager: this.keyboard_manager,
@@ -2146,10 +2144,8 @@ define([
      * 
      * @method load_notebook_success
      * @param {Object} data JSON representation of a notebook
-     * @param {String} status Description of response status
-     * @param {jqXHR} xhr jQuery Ajax object
      */
-    Notebook.prototype.load_notebook_success = function (data, status, xhr) {
+    Notebook.prototype.load_notebook_success = function (data) {
         var failed;
         try {
             this.fromJSON(data);
@@ -2281,20 +2277,18 @@ define([
      * Failure callback for loading a notebook from the server.
      * 
      * @method load_notebook_error
-     * @param {jqXHR} xhr jQuery Ajax object
-     * @param {String} status Description of response status
-     * @param {String} error HTTP error message
-     */
-    Notebook.prototype.load_notebook_error = function (xhr, status, error) {
-        this.events.trigger('notebook_load_failed.Notebook', [xhr, status, error]);
-        utils.log_ajax_error(xhr, status, error);
-        var msg = $("<div>");
-        if (xhr.status === 400) {
-            msg.text(utils.ajax_error_msg(xhr));
-        } else if (xhr.status === 500) {
-            msg.text("An unknown error occurred while loading this notebook. " +
+     * @param {Error} error
+     */
+    Notebook.prototype.load_notebook_error = function (error) {
+        this.events.trigger('notebook_load_failed.Notebook', error);
+        var msg;
+        if (error.name = utils.XHR_ERROR && error.xhr.status === 500) {
+            utils.log_ajax_error(error.xhr, error.xhr_status, error.xhr_error);
+            msg = "An unknown error occurred while loading this notebook. " +
             "This version can load notebook formats " +
-            "v" + this.nbformat + " or earlier. See the server log for details.");
+            "v" + this.nbformat + " or earlier. See the server log for details.";
+        } else {
+            msg = error.message;
         }
         dialog.modal({
             notebook: this,
@@ -2350,7 +2344,7 @@ define([
         var that = this;
         this.contents.list_checkpoints(this.notebook_path, this.notebook_name, {
             success: $.proxy(this.list_checkpoints_success, this),
-            error: function(xhr, status, error_msg) {
+            error: function(error) {
                 that.events.trigger('list_checkpoints_failed.Notebook');
             }
         });
@@ -2361,10 +2355,8 @@ define([
      * 
      * @method list_checkpoint_success
      * @param {Object} data JSON representation of a checkpoint
-     * @param {String} status Description of response status
-     * @param {jqXHR} xhr jQuery Ajax object
      */
-    Notebook.prototype.list_checkpoints_success = function (data, status, xhr) {
+    Notebook.prototype.list_checkpoints_success = function (data) {
         data = $.parseJSON(data);
         this.checkpoints = data;
         if (data.length) {
@@ -2384,7 +2376,7 @@ define([
         var that = this;
         this.contents.create_checkpoint(this.notebook_path, this.notebook_name, {
             success: $.proxy(this.create_checkpoint_success, this),
-            error: function (xhr, status, error_msg) {
+            error: function (error) {
                 that.events.trigger('checkpoint_failed.Notebook');
             }
         });
@@ -2395,10 +2387,8 @@ define([
      * 
      * @method create_checkpoint_success
      * @param {Object} data JSON representation of a checkpoint
-     * @param {String} status Description of response status
-     * @param {jqXHR} xhr jQuery Ajax object
      */
-    Notebook.prototype.create_checkpoint_success = function (data, status, xhr) {
+    Notebook.prototype.create_checkpoint_success = function (data) {
         data = $.parseJSON(data);
         this.add_checkpoint(data);
         this.events.trigger('checkpoint_created.Notebook', data);
@@ -2457,7 +2447,7 @@ define([
         this.contents.restore_checkpoint(this.notebook_path, this.notebook_name,
                                          checkpoint, {
             success: $.proxy(this.create_checkpoint_success, this),
-            error: function (xhr, status, error_msg) {
+            error: function (error) {
                 that.events.trigger('checkpoint_restore_failed.Notebook');
             }
         });
@@ -2467,11 +2457,8 @@ define([
      * Success callback for restoring a notebook to a checkpoint.
      * 
      * @method restore_checkpoint_success
-     * @param {Object} data (ignored, should be empty)
-     * @param {String} status Description of response status
-     * @param {jqXHR} xhr jQuery Ajax object
      */
-    Notebook.prototype.restore_checkpoint_success = function (data, status, xhr) {
+    Notebook.prototype.restore_checkpoint_success = function () {
         this.events.trigger('checkpoint_restored.Notebook');
         this.load_notebook(this.notebook_name, this.notebook_path);
     };
@@ -2488,8 +2475,8 @@ define([
         this.contents.delete_checkpoint(this.notebook_path, this.notebook_name, 
                                         checkpoint, {
             success: $.proxy(this.create_checkpoint_success, this),
-            error: function (xhr, status, error_msg) {
-                that.events.trigger('checkpoint_delete_failed.Notebook', [xhr, status, error]);
+            error: function (error) {
+                that.events.trigger('checkpoint_delete_failed.Notebook', error);
             }
         });
     };
@@ -2498,12 +2485,9 @@ define([
      * Success callback for deleting a notebook checkpoint
      * 
      * @method delete_checkpoint_success
-     * @param {Object} data (ignored, should be empty)
-     * @param {String} status Description of response status
-     * @param {jqXHR} xhr jQuery Ajax object
      */
-    Notebook.prototype.delete_checkpoint_success = function (data, status, xhr) {
-        this.events.trigger('checkpoint_deleted.Notebook', data);
+    Notebook.prototype.delete_checkpoint_success = function () {
+        this.events.trigger('checkpoint_deleted.Notebook');
         this.load_notebook(this.notebook_name, this.notebook_path);
     };
 
diff --git a/IPython/html/static/services/contents.js b/IPython/html/static/services/contents.js
index 7dda2a3f0..69190316a 100644
--- a/IPython/html/static/services/contents.js
+++ b/IPython/html/static/services/contents.js
@@ -21,12 +21,47 @@ define([
         this.base_url = options.base_url;
     };
 
+    /** Error type */
+    Contents.DIRECTORY_NOT_EMPTY_ERROR = 'DirectoryNotEmptyError';
+
+    Contents.DirectoryNotEmptyError = function() {
+        // Constructor
+        //
+        // An error representing the result of attempting to delete a non-empty
+        // directory.
+        this.message = 'A directory must be empty before being deleted.';
+    }
+    Contents.DirectoryNotEmptyError.prototype = new Error;
+    Contents.DirectoryNotEmptyError.prototype.name =
+        Contents.DIRECTORY_NOTE_EMPTY_ERROR;
+
+
     Contents.prototype.api_url = function() {
         var url_parts = [this.base_url, 'api/contents'].concat(
                                 Array.prototype.slice.apply(arguments));
         return utils.url_join_encode.apply(null, url_parts);
     };
 
+    /**
+     * Creates a basic error handler that wraps a jqXHR error as an Error.
+     *
+     * Takes a callback that accepts an Error, and returns a callback that can
+     * be passed directly to $.ajax, which will wrap the error from jQuery
+     * as an Error, and pass that to the original callback.
+     *
+     * @method create_basic_error_handler
+     * @param{Function} callback
+     * @return{Function}
+     */
+    Contents.prototype.create_basic_error_handler = function(callback) {
+        if (!callback) {
+            return function(xhr, status, error) { };
+        }
+        return function(xhr, status, error) {
+            callback(utils.wrap_ajax_error(xhr, status, error));
+        };
+    }
+
     /**
      * File Functions (including notebook operations)
      */
@@ -50,7 +85,7 @@ define([
             type : "GET",
             dataType : "json",
             success : options.success,
-            error : options.error || function() {}
+            error : this.create_basic_error_handler(options.error)
         };
         var url = this.api_url(path, name);
         $.ajax(url, settings);
@@ -71,7 +106,7 @@ define([
             type : "POST",
             dataType : "json",
             success : options.success || function() {},
-            error : options.error || function() {}
+            error : this.create_basic_error_handler(options.error)
         };
         $.ajax(this.api_url(path), settings);
     };
@@ -86,8 +121,12 @@ define([
             dataType : "json",
             success : options.success || function() {},
             error : function(xhr, status, error) {
-                utils.log_ajax_error(xhr, status, error);
-                error(xhr, status, error);
+                // TODO: update IPEP27 to specify errors more precisely, so
+                // that error types can be detected here with certainty.
+                if (xhr.status === 400) {
+                    error(new Contents.DirectoryNotEmptyError());
+                }
+                error(utils.wrap_ajax_error(xhr, status, error));
             }
         };
         var url = this.api_url(path, name);
@@ -103,8 +142,8 @@ define([
             data : JSON.stringify(data),
             dataType: "json",
             contentType: 'application/json',
-            success : options.success || function() {}, 
-            error : options.error || function() {}
+            success : options.success || function() {},
+            error : this.create_basic_error_handler(options.error)
         };
         var url = this.api_url(path, name);
         $.ajax(url, settings);
@@ -119,7 +158,7 @@ define([
             data : JSON.stringify(model),
             contentType: 'application/json',
             success : options.success || function() {},
-            error : options.error || function() {}
+            error : this.create_basic_error_handler(options.error)
         };
         if (options.extra_settings) {
             $.extend(settings, options.extra_settings);
@@ -137,7 +176,7 @@ define([
         var settings = {
             type : "POST",
             success: options.success || function() {},
-            error: options.error || function() {}
+            error : this.create_basic_error_handler(options.error)
         };
         $.ajax(url, settings);
     };
@@ -147,7 +186,7 @@ define([
         var settings = {
             type : "GET",
             success: options.success,
-            error: options.error || function() {}
+            error : this.create_basic_error_handler(options.error)
         };
         $.ajax(url, settings);
     };
@@ -157,17 +196,17 @@ define([
         var settings = {
             type : "POST",
             success: options.success || function() {},
-            error: options.error || function() {}
+            error : this.create_basic_error_handler(options.error)
         };
         $.ajax(url, settings);
     };
 
-    Contents.prototype.delete_checkpoint = function(path, name, checkpoint_id, options) {        
+    Contents.prototype.delete_checkpoint = function(path, name, checkpoint_id, options) {
         var url = this.api_url(path, name, 'checkpoints', checkpoint_id);
         var settings = {
             type : "DELETE",
             success: options.success || function() {},
-            error: options.error || function() {}
+            error : this.create_basic_error_handler(options.error)
         };
         $.ajax(url, settings);
     };
@@ -199,7 +238,7 @@ define([
             type : "GET",
             dataType : "json",
             success : options.success,
-            error : options.error || function() {}
+            error : this.create_basic_error_handler(options.error)
         };
 
         $.ajax(this.api_url(path), settings);
@@ -209,4 +248,4 @@ define([
     IPython.Contents = Contents;
 
     return {'Contents': Contents};
-}); 
+});
diff --git a/IPython/html/static/tree/js/main.js b/IPython/html/static/tree/js/main.js
index 1f53a7f77..73ed8b18e 100644
--- a/IPython/html/static/tree/js/main.js
+++ b/IPython/html/static/tree/js/main.js
@@ -62,23 +62,17 @@ require([
     $('#new_notebook').click(function (e) {
         contents.new_notebook(common_options.notebook_path,
             {
-                success: function (data, status, xhr) {
+                success: function (data) {
                     window.open(
                         utils.url_join_encode(
                             common_options.base_url, 'notebooks',
                             data.path, data.name
                         ), '_blank');
                     },
-                error: function(xhr, status, error) {
-                    var msg;
-                    if (xhr.responseJSON && xhr.responseJSON.message) {
-                        msg = xhr.responseJSON.message;
-                    } else {
-                        msg = xhr.statusText;
-                    }
+                error: function(error) {
                     dialog.modal({
                         title : 'Creating Notebook Failed',
-                        body : "The error was: " + msg,
+                        body : "The error was: " + error.message,
                         buttons : {'OK' : {'class' : 'btn-primary'}}
                     });
                 }
diff --git a/IPython/html/static/tree/js/notebooklist.js b/IPython/html/static/tree/js/notebooklist.js
index 2e62f40b5..a044025e2 100644
--- a/IPython/html/static/tree/js/notebooklist.js
+++ b/IPython/html/static/tree/js/notebooklist.js
@@ -144,9 +144,8 @@ define([
         var that = this
         this.contents.list_contents(that.notebook_path, {
             success: $.proxy(this.draw_notebook_list, this),
-            error: function(xhr, status, error) { 
-                utils.log_ajax_error(xhr, status, error);
-                that.draw_notebook_list([], "Error connecting to server.");
+            error: function(error) {
+                that.draw_notebook_list([], "Server error: " + error.message);
             }
         });
     };