Add error handling

backend/app.js

@@ -9,4 +9,8 @@ app.use(express.json());
 
 app.use('/v1/notes', notesRoutes);
 
+app.use((req, res) => {
+   res.status(404).json({ error: `Requested path is invalid or does not exist: ${req.method} ${req.originalUrl}` });
+ });
+
 export default app;