backport WebSocket.send_error from tornado 4.1

raising an exception in a websocket results in
"Method not supported" on tornado 4.0

IPython/html/base/zmqhandlers.py

@@ -94,6 +94,19 @@ if os.environ.get('IPYTHON_ALLOW_DRAFT_WEBSOCKETS_FOR_PHANTOMJS', False):
 
 class ZMQStreamHandler(WebSocketHandler):
     
+    if tornado.version_info < (4,1):
+        """Backport send_error from tornado 4.1 to 4.0"""
+        def send_error(self, *args, **kwargs):
+            if self.stream is None:
+                super(WebSocketHandler, self).send_error(*args, **kwargs)
+            else:
+                # If we get an uncaught exception during the handshake,
+                # we have no choice but to abruptly close the connection.
+                # TODO: for uncaught exceptions after the handshake,
+                # we can close the connection more gracefully.
+                self.stream.close()
+
+
     def check_origin(self, origin):
         """Check Origin == Host or Access-Control-Allow-Origin.