Fix an issues with moved Phriction documents if the old location does not exist or can't be loaded

Summary: Fixes T5250. This needs some general cleanup, but fix the fatal.

Test Plan:
  - Viewed moved document.
  - Viewed moved-from-nonexistent-source document.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5250

Differential Revision: https://secure.phabricator.com/D9357