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