Explicitly check for supported VCS

Summary: Instead of having an ArcanistWorkflow subclass explicitly throw an exception when run in an unsupported VCS, consolidate this code and move it to arcanist.php. In doing so, we lose some specificity in some of the error messages, but this otherwise feels cleaner. We could consider adding a getUnsupportedRevisionControlSystemMessage() method to provide a more tailored error message. Depends on D11604.

Test Plan:
Ran arc bookmark in a git working copy:

Usage Exception: `arc bookmark` is only supported under hg.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11550