Don't try to synchronize repositories with no working copy

Summary:
Ref T4292. Sometimes, we may not have a working copy for a repository. The easiest way to get into this condition is to deactivate a repository.

We could try to clone + fetch in this case, but that's kind of complex, and there's an easy command that administrators can run manually. For now, just tell them to do that.

This affects the inactive repositories on secure, like rGITCOINS.

Test Plan: Removed working copy, got message.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4292

Differential Revision: https://secure.phabricator.com/D15786