Check that all po/pot files are valid

Perform minimal checks on po and pot files so that no broken files can
get imported.

Change-Id: I9b5edbad9f4302ca24e4255f503ee2db45ebd5d7
Co-Authored-By: Clark Boylan <clark.boylan@gmail.com>
Related-Bug: #1299349
Merge "Check that all po/pot files are valid"