ConnectionError for gated datasets and unauthenticated users

[albertvillanova]

Since the Hub returns dataset info for gated datasets and unauthenticated users, there is dead code:

datasets/src/datasets/load.py

Lines 1846 to 1852 in 98fdc9e

	except GatedRepoError as e:
	message = f"Dataset '{path}' is a gated dataset on the Hub."
	if "401 Client Error" in str(e):
	message += " You must be authenticated to access it."
	elif "403 Client Error" in str(e):
	message += f" Visit the dataset page at https://huggingface.co/datasets/{path} to ask for access."
	raise DatasetNotFoundError(message) from e

We should remove the dead code and properly handle this case: currently we are raising a ConnectionError instead of a DatasetNotFoundError (as before).

See:

• CI worker tests are broken for gated datasets: ConnectionError dataset-viewer#3025
• Gated dataset info is leaked huggingface_hub#2457