Improve some messaging in `arc which` for autodetection of repositories

Summary:
  - The modern name for the config is "project.name".
  - Missing parameter in a pht().
  - When the value is set, but not valid, we gave you a misleading error message.

Test Plan: Ran `arc which`.

Reviewers: talshiri, btrahan

Reviewed By: talshiri

CC: aran

Differential Revision: https://secure.phabricator.com/D8081