Do safe checking on exception message

src/background/messages/makeRequest.ts

@@ -94,7 +94,7 @@ const handler: PlasmoMessaging.MessageHandler<Request, Response<any>> = async (r
     console.error('failed request', err);
     res.send({
       success: false,
-      error: err.message,
+      error: err instanceof Error ? err.message : err,
     });
   }
 };