Improve handling of bad branches in Diffusion
Summary:
Fixes T7972.
- Trap the RefNotFound error which may occur in getAlternatives().
- Improve error handling in Mercurial.
Test Plan: {F387611}
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T7972
Differential Revision: https://secure.phabricator.com/D12590