Getting DjangoTranslation that doesn't exist should return None

[Sander van Leeuwen]

Getting a DjangoTranslation that doesn't exist used to return None.

Since ​this commit it raises an IOError when using an unavailable language.

​This test suggests this is wanted behaviour.

But it should be possible to add a language that is not available in django/conf/locale/ right?

To reproduce, use a LANGUAGE_CODE unavailable in django/conf/locale/.

For instance:
LANGUAGE_CODE = 'sd-PK'

run python manage.py validate

This will raise:
IOError: [Errno 2] No translation file found for domain: u'django'