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