Add a Client class to json_rpc_2.

pkg/json_rpc_2/lib/src/exception.dart

 // Copyright (c) 2014, 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 json_rpc_2.exception;
 
 import '../error_code.dart' as error_code;
 
 /// An exception from a JSON-RPC server that can be translated into an error
 /// response.
@@ skipping 46 matching lines @@
     }
 
     var id = request is Map ? request['id'] : null;
     if (id is! String && id is! num) id = null;
     return {
       'jsonrpc': '2.0',
       'error': {'code': code, 'message': message, 'data': modifiedData},
       'id': id
     };
   }
+
+  String toString() {
+    var prefix = "JSON-RPC error $code";
+    var errorName = error_code.name(code);
+    if (errorName != null) prefix += " ($errorName)";
+    return "$prefix: $message";
+  }
 }