Fix ConnectionError for gated datasets and unauthenticated users
Fix `ConnectionError` for gated datasets and unauthenticated users. See:
- https://github.com/huggingface/dataset-viewer/issues/3025

Note that a recent change in the Hub returns dataset info for gated datasets and unauthenticated users, instead of raising a `GatedRepoError` as before. See:
- https://github.com/huggingface/huggingface_hub/issues/2457

This PR adds an additional check (/auth-check) for gated datasets and raises `DatasetNotFoundError` for unauthenticated users, as it was the case before the change in the Hub.
- Fix suggested by @Pierrci (thanks @Wauplin for pointing it out).

Fix #7109.