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 `checkForCancellation()`.
  - Ran a failing build where the final step caused the failure.
  - Observed `phlog()` in `bin/harbormaster` output.
  - Observed log in web UI:

{F101168}

Reviewers: btrahan, hach-que

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2015

Differential Revision: https://secure.phabricator.com/D7935