load widget extension in js

previously was a plain require,
which doesn't actually register anything.

notebook/static/notebook/js/main.js

@@ -51,10 +51,8 @@ require([
     "use strict";
 
     // BEGIN HARDCODED WIDGETS HACK
-    // Don't include in above require call, avoiding r.js
-    // and crash on import failure.
-    require(['nbextensions/widgets/notebook/js/extension'], function(){}, function(err) {
-        console.error('ipywidgets package not installed.  Widgets are not available.');
+    utils.load_extension('widgets/notebook/js/extension').catch(function () {
+        console.warn('ipywidgets package not installed.  Widgets are not available.');
     });
     // END HARDCODED WIDGETS HACK