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