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