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