Fixing memory leak (#740)

handle connection closed better

* This improves memory efficiency and closed connection reporting.

Author: ChrisPaulBennett

cylc/uiserver/graphql/tornado_ws.py

@@ -106,9 +106,10 @@ async def receive(self):
             raise ConnectionClosedException()
 
     async def send(self, data):
-        if self.closed:
-            return
-        await self.ws.write_message(data)
+        if self.ws.ws_connection and not self.ws.ws_connection.is_closing():
+            await self.ws.write_message(data)
+        else:
+            raise WebSocketClosedError
 
     @property
     def closed(self):