[WIP] Add check for untranslated plural targets.

[unho]

This only reports failure if either:

• There is any plural target translated while there is any other untranslated,
• The unit's plural target count differs from the language's nplural.

The second reason is because because otherwise no error would be reported if for example a unit has more plural targets than its language's nplurals, but only has translated exactly the same number of plural targets as the language's nplurals.

Fixes #3598.

[dwaynebailey]

Please review comments, otherwise lgtm