Rework error handling in the service protocol and in Observatory.

runtime/observatory/lib/src/elements/action_link.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library action_link_element;
 
+import 'observatory_element.dart';
 import 'package:polymer/polymer.dart';
 
 @CustomTag('action-link')
-class ActionLinkElement extends PolymerElement {
+class ActionLinkElement extends ObservatoryElement {
   ActionLinkElement.created() : super.created();
 
   @observable bool busy = false;
   @published var callback = null;
   @published String label = 'action';
   @published String color = null;
 
   void doAction(var a, var b, var c) {
     if (busy) {
       return;
     }
     if (callback != null) {
       busy = true;
-      // TODO(turnidge): Track down why adding a dummy argument makes
-      // this work but having a no-argument callback doesn't.
-      callback(null).whenComplete(() {
+      callback()
+        .catchError(app.handleException)
+        .whenComplete(() {
           busy = false;
         });
     }
   }
 }