R1:1bf68e06a501

Improve Diffusion error messages and UI for partially imported repositories

Summary:
- When you have an un-cloned repository, we currently throw random-looking Git/Hg exception. Instead, throw a useful error.
- When you have a cloned but undiscovered repository, we show no commits. This is crazy confusing. Instead, show commits as "importing...".
- Fix some warnings and errors for empty path table cases, etc.

Test Plan:
- Wiped database.
- Added Mercurial repo without running daemons. Viewed in…