Fix plural handling, put teeth in msgfmt -c test.
More validation of .po files: check message structure

The tests in test_validate.py now examine the messages to see that they
have the right structure.  The English and the translation should have
the same tags, the translation can't be blank, and it can't have any
Astral plane characters.
Say a little about what's happening when pulling from transifex
Better handling of plurals during .po validation

Now any of the plurals being missing will count as an Empty translation,
and for looking at tags, the two English strings and all the translated
strings are mooshed together.