#tf-data-service Explicitly checks if snapshot metadata exists.
#tf-data-service Explicitly checks if snapshot metadata exists.

The `GFile` documentation does not mention what exceptions it raises:
https://github.com/tensorflow/tensorflow/blob/38b17d708344a91234dad879794735e79f9af42a/tensorflow/python/platform/gfile.py#L37.

It would be safer to explicitly check for file existence and return the
appropriate result.
