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: I328fc11c688c16737ae88984e4778483ce5db5ab
Co-Authored-By: Clark Boylan <clark.boylan@gmail.com>
Related-Bug: #1298645
Merge "Check that all po/pot files are valid"