Fix error behavior of "arc version" when it encounters a library which is not a working copy

Summary:
Ref T13504. The API has changed here slightly, and if you run "arc version" without "arcanist/" being a Git working copy, it currently fatals in a misleading way.

Instead, reach the error properly.

Test Plan: Ran "arc version" after moving aside ".git/", got a helpful error message instead of a confusing "call on null" exception.

Maniphest Tasks: T13504

Differential Revision: https://secure.phabricator.com/D21050