Rework error handling in the service protocol and in Observatory.

runtime/observatory/tests/service/get_retaining_path_rpc_test.dart

 // Copyright (c) 2015, 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.
 // VMOptions=--compile-all --error_on_bad_type --error_on_bad_override
 
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 
 import 'test_helper.dart';
 
@@ skipping 38 matching lines @@
     var result = await isolate.invokeRpcNoUpgrade('_getRetainingPath', params);
     expect(result['elements'][1]['value']['name'], equals('globalObject'));
   },
 
   // missing limit.
   (Isolate isolate) async {
     var obj = await eval(isolate, 'globalObject');
     var params = {
       'targetId': obj['id'],
     };
-    var result = await isolate.invokeRpcNoUpgrade('_getRetainingPath', params);
-    expect(result['message'], contains("invalid 'limit' parameter"));
+    bool caughtException;
+    try {
+      await isolate.invokeRpcNoUpgrade('_getRetainingPath', params);
+      expect(false, isTrue, reason:'Unreachable');
+    } on ServerRpcException catch (e) {
+      caughtException = true;
+      expect(e.code, equals(ServerRpcException.kInvalidParams));
+      expect(e.data['details'],
+             "_getRetainingPath expects the \'limit\' parameter");
+    }
+    expect(caughtException, isTrue);
   },
 
   (Isolate isolate) async {
     var target1 = await eval(
         isolate, '() { var tmp = target1; target1 = null; return tmp;} ()');
     var params = {
       'targetId': target1['id'],
       'limit': 4,
     };
     var result = await isolate.invokeRpcNoUpgrade('_getRetainingPath', params);
@@ skipping 23 matching lines @@
       'limit': 4,
     };
     var result = await isolate.invokeRpcNoUpgrade('_getRetainingPath', params);
     expect(result['type'], equals('RetainingPath'));
     expect(result['elements'][0]['parentListIndex'], equals(12));
     expect(result['elements'][2]['value']['name'], equals('globalList'));
   },
 ];
 
 main(args) async => runIsolateTests(args, tests, testeeBefore:warmup);