Change all js throw string to throw error in notebook/static

notebook/static/base/js/keyboard.js

@@ -325,7 +325,7 @@ define([
             if(typeof(current_node) === 'string'){
                 delete tree[shortcut_array[0]];
             } else {
-                throw('try to delete non-leaf');
+                throw new Error('try to delete non-leaf');
             }
         } else {
             this._remove_leaf(shortcut_array.slice(1),  tree[shortcut_array[0]], allow_node);

notebook/static/base/js/notificationarea.js

@@ -52,7 +52,7 @@ define([
      */
     NotificationArea.prototype.get_widget = function (name) {
         if(this.widget_dict[name] === undefined) {
-            throw('no widgets with this name');
+            throw new Error('no widgets with this name');
         }
         return this.widget_dict[name];
     };
@@ -66,7 +66,7 @@ define([
      */
     NotificationArea.prototype.new_notification_widget = function (name) {
         if (this.widget_dict[name] !== undefined) {
-            throw('widget with that name already exists!');
+            throw new Error('widget with that name already exists!');
         }
 
         // create the element for the notification widget and add it

notebook/static/notebook/js/actions.js

@@ -686,7 +686,7 @@ define(function(require){
             data = {handler:data};
         }
         if(typeof(data.handler) !== 'function'){
-            throw('unknown datatype, cannot register');
+            throw new Error('unknown datatype, cannot register');
         }
         var _data = data;
         data = {};