Check that all po/pot files are valid

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

Add msgfmt to list of requirements in doc/source/setup.rst.

Change-Id: Ifa9fe2fc1ca9a41621ff3039c249700cdb1cd0ac
Related-Bug: #1299349
Co-Authored-By: Clark Boylan <clark.boylan@gmail.com>
Merge "Check that all po/pot files are valid"