Catch exception in differential

Summary:
This masks possible configuration issues and slightly degrades functionality
with the tradeoff of having differential work when phabricator isn't quite
configured correctly.

Test Plan:
remove directory for a repository, load differential revision from that repo,
and see differential load.

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2512

Differential Revision: https://secure.phabricator.com/D4859