fix connection exception cause high cpu load

adjust err log

Author: chengcong

jupyter_server/gateway/connections.py

@@ -147,6 +147,9 @@ def handle_incoming_message(self, message: str) -> None:
         """Send message to gateway server."""
         if self.ws is None and self.ws_future is not None:
             loop = IOLoop.current()
+            if self.ws_future.done():
+                self.log.error(f"Exception connect to  gateway server {self.ws_future.exception()}")
+                return
             loop.add_future(self.ws_future, lambda future: self.handle_incoming_message(message))
         else:
             self._write_message(message)