Add additional protections against local repository misconfigurations

Summary: We've hit a couple of these in the wild, raise better error messages when the local repo is toast / broken / nonsense.

Test Plan: Broke my local repo in all of the different ways we test for, verified I got an error message in each case.

Reviewers: btrahan, abirchall

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T964, T924

Differential Revision: https://secure.phabricator.com/D1855