Fail with useful failure messages on genuine failures.

References TODO in astroquery/fermi/core (122).

class GetFermilatDatafile(object):
"""
TODO: document
TODO: Fail with useful failure messages on genuine failures
(this doesn't need to be implemented as a class)
"""

Start in astroquery/fermi/core at the GetFermilatDatafile class and its TODO comments. Determine which genuine failures need distinct messages, then verify that those failures report useful errors without masking other behavior.

Written by the indexing model from the issue text.