Bump Phabricator server version to 3

Summary: See D1257. Also make the error message more friendly, and remove a very
very old Facebook-specific error.

Test Plan:

• Tried to diff with an older arc.
• Tried to diff with a newer arc.
• Diffed with the right arc.

Reviewers: btrahan, jungejason, aran

Reviewed By: aran

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1258