[execution][local] Improve handling of message handler shutdown

Relates to beat.backend.python#30

beat/core/execution/local.py

@@ -200,7 +200,12 @@ class LocalExecutor(BaseExecutor):
         for handler in [self.message_handler, self.loop_message_handler]:
             if handler:
                 handler.kill()
-                handler.join()
+                try:
+                    handler.join()
+                except RuntimeError:
+                    # The handler was not started
+                    pass
+
                 handler.destroy()
 
         for socket in [self.executor_socket, self.loop_socket]: