Improve error message if local Git working copy directory exists but isn't a working copy

Summary: Fixes T9701. I don't want to try to autofix this because destroying the directory could destroy important files, but we can improve the error message.

Test Plan: Faked a failure, ran repository update X, got a more tailored error message.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9701

Differential Revision: https://secure.phabricator.com/D14971