Add appengine/gcloud/mustache dependencies.

pkg/appengine/lib/api/errors.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 appengine.api.errors;
+
+import 'dart:io';
+
+class AppEngineError implements Exception {
+  final String message;
+
+  AppEngineError(this.message);
+
+  String toString() => "AppEngineException: $message";
+}
+
+class NetworkError extends AppEngineError implements IOException {
+  NetworkError(String message) : super(message);
+
+  String toString() => "NetworkError: $message";
+}
+
+class ProtocolError extends AppEngineError implements IOException {
+  static ProtocolError INVALID_RESPONSE = new ProtocolError("Invalid response");
+
+  ProtocolError(String message) : super(message);
+
+  String toString() => "ProtocolError: $message";
+}
+
+class ServiceError extends AppEngineError {
+  final String serviceName;
+
+  ServiceError(String message, {this.serviceName: 'ServiceError'})
+      : super(message);
+
+  String toString() => "$serviceName: $message";
+}
+
+class ApplicationError extends AppEngineError {
+  ApplicationError(String message) : super(message);
+
+  String toString() => "ApplicationError: $message";
+}