Return error if fetching nonexistent reference

[clearydude]

In the method that validates the correctness of reference locations, all_push_entries_in_fetch_head, it currently fails to return an error if one of the refs we attempted to fetch does not exist.
This should return an error, since git will return a fatal error when attempting to fetch a nonexistent reference. The current non-failure mode may potentially cause the RSL implementation to fail to detect deletion attacks that git DOES detect.