Tidy up some low-level exception handling
- Use OSError with EPERM as our error condition when being run as root. SystemExit is really designed for stuff that calls sys.exit() and other low-level stuff. OSError is a far better description of what's happening here.
- Remove weird _handle_system_exit() abstraction. I'm not even sure why we need this since none of our code outside of here calls sys.exit() or raises this exception, but at least it's slighly less confusing now
Reviewers: mmodell, Release-Engineering-Team, thcipriani
Reviewed By: Release-Engineering-Team, thcipriani
Subscribers: bd808, thcipriani, jenkins
Differential Revision: https://phabricator.wikimedia.org/D795