Clarify error message when the loop is already running

Author: 1st1

asyncio/base_events.py

@@ -393,7 +393,7 @@ def run_forever(self):
         """Run until stop() is called."""
         self._check_closed()
         if self.is_running():
-            raise RuntimeError('Event loop is running.')
+            raise RuntimeError('This event loop is already running')
         if events._get_running_loop() is not None:
             raise RuntimeError(
                 'Cannot run the event loop while another loop is running')