R1:996930da2ae0

Improve several exception behaviors for Harbormaster workers

Summary:
Ref T2015. Several fixes:

- `checkForCancellation()` no longer exists, and isn't relevant for resumable stops. Throw it away for now.
- Fix an issue where a build could pass even if the final step failed.
- `phlog()` exceptions so they show up in `bin/harbormaster` and the daemon logs.
- Write an exception log if a step fails.
- Add a "throw an exception" step to debug this stuff more easily.

Test Plan:
- Grepped for…