Summary:
- Allow new-style setup to raise fatal setup errors.
- Port extension checks to new-style setup as fatal errors.
- When fatal errors are raised, abort setup and show them in a chrome-free response.
Test Plan: {F29981}
Reviewers: btrahan, chad
Reviewed By: chad
CC: aran
Maniphest Tasks: T2228
Differential Revision: https://secure.phabricator.com/D4587
