Port extension checks to new-style setup

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