[MD Extensions] Include the manifest code snippet in load errors

chrome/browser/resources/md_extensions/service.js

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 cr.define('extensions', function() {
   'use strict';
 
   /**
    * @constructor
    * @implements {extensions.ErrorPageDelegate}
@@ skipping 235 matching lines @@
     },
 
     /** @override */
     setProfileInDevMode: function(inDevMode) {
       chrome.developerPrivate.updateProfileConfiguration(
           {inDeveloperMode: inDevMode});
     },
 
     /** @override */
     loadUnpacked: function() {
-      chrome.developerPrivate.loadUnpacked({failQuietly: true}, () => {
-        // TODO(devlin): Show the load error dialog if something went wrong.
+      chrome.developerPrivate.loadUnpacked(
+          {failQuietly: true, populateError: true},
+          (loadError) => {
+        if (chrome.runtime.lastError &&
+            chrome.runtime.lastError.message !=
+                'File selection was canceled.') {
+          throw new Error(chrome.runtime.lastError.message);
+        }
+        if (loadError) {
+          this.manager_.loadError.set('loadError', loadError);
+          this.manager_.loadError.show();
+        }
       });
     },
 
     /** @override */
     retryLoadUnpacked: function() {
       // TODO(devlin): Implement this.
     },
 
     /** @override */
     choosePackRootDirectory: function() {
@@ skipping 35 matching lines @@
           resolve(code);
         });
       });
     },
   };
 
   cr.addSingletonGetter(Service);
 
   return {Service: Service};
 });