Add a better error message for unknown widgets

core/modules/widgets/error.js

@@ -0,0 +1,41 @@
+/*\
+title: $:/core/modules/widgets/error.js
+type: application/javascript
+module-type: widget
+
+The error widget displays an error message.
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+var ErrorWidget = function(renderer,errorMessage) {
+	// Save state
+	this.renderer = renderer;
+	this.errorMessage = errorMessage;
+	// Generate child nodes
+	this.generateChildNodes();
+};
+
+ErrorWidget.prototype.generateChildNodes = function() {
+	// Create the wrapper node
+	var node = {
+		type: "element",
+		tag: "span",
+		children: [{
+			type: "text",
+			text: this.errorMessage
+		}]
+	};
+	// Set up the attributes for the wrapper element
+	$tw.utils.addClassToParseTreeNode(node,"tw-error-widget");
+	// Create the renderers for the wrapper and the children
+	this.children = this.renderer.renderTree.createRenderers(this.renderer.renderContext,[node]);
+};
+
+exports.error = ErrorWidget;
+
+})();