Let `arc liberate` throw on unsupported PHP features

Summary: Ref T4725.

Test Plan: add some files that have unsupported constructs all over the place and run `arc liberate`

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4725

Differential Revision: https://secure.phabricator.com/D9585