Fix a minor i18n issue in libphutil

Summary:
Ref T5267. Fixes a missing %s.

Also ignores the src/.cache/ directory, used for the i18n cache.

Test Plan: Extracted strings more cleanly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5267

Differential Revision: https://secure.phabricator.com/D16809