Instead of having an ArcanistWorkflow 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() function to provide a more tailored error message. Depends on D11549

Ran arc bookmark in a git working copy:

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