Summary: Ref T5936. This implements build implementations aborting early when the build has since been restarted. Build steps now periodically poll to see if the build's current generation does not match their generation, and they throw a `HarbormasterBuildAbortedException` if that is the case.
Test Plan: Tested locally on my machine with the sleep build step.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: epriestley, Korvin
Maniphest Tasks: T5936
Differential Revision: https://secure.phabricator.com/D10322
Summary: We've received feedback that the "core - exception" is incredibly confusing, to the point where developers see this and write off the build failure as a Phabricator error that is unrelated to their changes.
Test Plan: Ran a build with a `exit 1` run step, didn't see the "core - exception" appear.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D10090
