R1:f08908ff352f

Raise a setup warning for missing or invalid local repository directory

Summary: I'm planning to add more detailed info to Diffusion itself, but catch the big issue here.

Test Plan: Hit config issue locally, then resolved it.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7439