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: I5466178f713bfea3c4a674e1b408a060b3a526c5
Co-Authored-By: Clark Boylan <clark.boylan@gmail.com>
Related-Bug: #1299349
Merge "Check that all po/pot files are valid"