Add user agent to ClientError log message

This would help to keep track of which errors are relevant to the latest 
client build and which can be ignored because they have already been 
fixed or are coming from an alternate client implementation.

server/lobbyconnection.py

@@ -189,7 +189,11 @@ async def on_message_received(self, message):
             })
             await self.abort(e.message())
         except ClientError as e:
-            self._logger.warning("Client error: %s", e.message)
+            self._logger.warning(
+                "ClientError[%s]: %s",
+                self.user_agent,
+                e.message,
+            )
             await self.send({
                 "command": "notice",
                 "style": "error",