Make messages translatable and more sensible.

Summary:
These exception messages & comments didn't quite match reality.
Fixed and added pht() around a couple of them.

Test Plan: I didn't test this :P

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17578