docs(examples): fix error connection handling (#40633)

When there's an error while connect to mongodb (timeout for example), it will be cached and need to restart next.js, by handling the error correctly (clear cached promise) it will try to reconnect to mongodb if the function is called again

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)


Co-authored-by: JJ Kasper <[email protected]>

examples/with-mongodb-mongoose/lib/dbConnect.js

@@ -33,7 +33,14 @@ async function dbConnect() {
       return mongoose
     })
   }
-  cached.conn = await cached.promise
+
+  try {
+    cached.conn = await cached.promise
+  } catch (e) {
+    cached.promise = null
+    throw e
+  }
+
   return cached.conn
 }