Adds dialog on new_notebook failure

This logic doesn't really belong in ContentManager.  It would be better to trigger an event, which is handled somewhere else.  But there's no obvious place to put this event, so creating the error dialog inside the new_notebook method is ok for now.

IPython/html/static/base/js/contentmanager.js

@@ -5,7 +5,8 @@ define([
     'base/js/namespace',
     'jquery',
     'base/js/utils',
-], function(IPython, $, utils) {
+    'base/js/dialog',
+], function(IPython, $, utils, dialog) {
     var ContentManager = function(options) {
         // Constructor
         //
@@ -50,7 +51,20 @@ define([
                     '_blank'
                 );
             },
-            error : utils.log_ajax_error,
+            error : function(xhr, status, error) {
+                utils.log_ajax_error(xhr, status, error);
+                var msg;
+                if (xhr.responseJSON && xhr.responseJSON.message) {
+                    msg = xhr.responseJSON.message;
+                } else {
+                    msg = xhr.statusText;
+                }
+                dialog.modal({
+                    title : 'Creating Notebook Failed',
+                    body : "The error was: " + msg,
+                    buttons : {'OK' : {'class' : 'btn-primary'}}
+                });
+            }
         };
         var url = utils.url_join_encode(
             base_url,