Fix the error response for bad isolate ids, etc.

runtime/vm/service/message.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.
 
 part of vmservice;
 
 class Message {
   final Completer _completer = new Completer.sync();
   bool get completed => _completer.isCompleted;
   /// Future of response.
@@ skipping 121 matching lines @@
         ..[4] = keys
         ..[5] = values;
     sendRootServiceMessage(request);
     return _completer.future;
   }
 
   void setResponse(String response) {
     _completer.complete(response);
   }
 
-  void setErrorResponse(String message) {
-    var response = {
-      'jsonrpc': '2.0',
-      'id': serial,
-      'result' : {
-        'type': 'Error',
-        'message': message,
-        'request': {
-          'method': method,
-          'params': params
-        }
-      }
-    };
-    _completer.complete(JSON.encode(response));
+  void setErrorResponse(int code, String details) {
+    _completer.complete(encodeRpcError(this, code,
+                                       details: '$method: $details'));
   }
 }
 
 bool sendIsolateServiceMessage(SendPort sp, List m)
     native "VMService_SendIsolateServiceMessage";
 
 void sendRootServiceMessage(List m)
     native "VMService_SendRootServiceMessage";