Tweak error and status messages for commit ranges

Summary: Improve/clarify some error messages a bit, hopefully.

Test Plan: Ran `arc which`, `arc diff`, etc., with various explicit, implicit, and `--head` flags. Read error messages, didn't catch anything too awkward.

Reviewers: talshiri

Reviewed By: talshiri

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9489