Make sure exceptions are surfaced from setup

Summary: If setup throws an exception, we may swallow it currently. Make sure it's printed.

Test Plan: Changed "git" to "qit" to force a command failure, ran setup, got a more useful error.

Reviewers: btrahan, vrana, jungejason

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D2197