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 Diffusion, got a good exception.
• Pulled Mercurial repo without discovering it. Got "Importing...".
• Discovered Mercurial repo without parsing it. Got "Importing..." plus date information.
• Parsed Mercurial repo, got everything working properly.
• Added Git repo without running daemons, did all the stuff above, same results.
• This doesn't improve SVN much but that's a trickier case since we don't actually make SVN calls and rely only on the parse state.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T776

Differential Revision: https://secure.phabricator.com/D2439