This should mimic the behavior of most of the [other `get_` methods](https://github.com/buddylindsey/django-braces/blob/csvresponse-mixin/braces/views/_other.py#L23). It should raise an exception if the filename isn't set and the method isn't overwritten.

Also, can we rename it to `get_csv_filename` since `csv_filename` is the class attribute?