Throw a more tailored exception after failing to resolve a ref

Summary: Ref T2683. Throw a more tailored exception to allow callers to distinguish between bad refs (which are expected, if users try to visit garbage branches) and other types of errors.

Test Plan: Tried to view branch "alksndfklansdf". Viewed branch "master".

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2683

Differential Revision: https://secure.phabricator.com/D9094