Improve exception behavior for Herald commit rules which fail to load diff context

Summary:
This code is a little funky right now, and can return `array("error message")` and then try to call `getHunks()` on it. Additionally, each field loads the commit's changes separately.

Instead, load the commit's changes once and cache them, and handle exceptions appropriately.

Test Plan:
  - Created a rule like "changed, added, removed content all match /.*/" to force all fields to generate.
  - Ran it successfully.
  - Faked an error and ran it, got reasonable results.

Reviewers: btrahan

Reviewed By: btrahan

CC: bigo, aran

Differential Revision: https://secure.phabricator.com/D7384