Copy the web socket stuff from http_parser.

lib/src/exception.dart

+// Copyright (c) 2016, 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.
+
+import 'channel.dart';
+
+/// An exception thrown by a [WebSocketChannel].
+class WebSocketChannelException implements Exception {
+  final String message;
+
+  /// The exception that caused this one, if available.
+  final inner;
+
+  WebSocketChannelException([this.message, this.inner]);
+
+  String toString() => message == null
+      ? "WebSocketChannelException" :
+        "WebSocketChannelException: $message";
+}