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