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: I5dd0ca872fa25f8ef9557f039904090c4a672ff3
Co-Authored-By: Clark Boylan <clark.boylan@gmail.com>
Related-Bug: #1299349
Merge "Check that all po/pot files are valid"