Allow build steps to explicitly fail the build
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