Resolve "Look into error coming from Gitaly in ListAgentConfigFiles"

See #448 (closed) for details.

I don't think that it's worth the effort to actually verify that the repository is empty or the agent configuration directory doesn't exist when we get an InvalidArgument error from Gitaly. I think it's good enough to just check the Gitaly error message (unfortunately, there is no dedicated error / status code for this).

Closes #448 (closed)