R1:2c3d071a26a9

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…