Updated FormSet.non_form_errors() to ensure full_clean() has
been called before returning the errors.

The fixture named "initial_data" is exceptional though; if it
doesn't exist, the error is not raised. This allows syncdb and
flush management commands to attempt to load it without causing
an error if it doesn't exist.

If the fixture doesn't exist, loaddata will output a warning.

The fixture named "initial_data" is exceptional though; if it
doesn't exist, the warning is not emitted. This allows syncdb and
flush management commands to attempt to load it without causing
spurious warnings.

Thanks to Derega, ptone, dirigeant and d1ffuz0r for contributions
to the ticket.

Fixed #18990: Loaddata now complains if fixture doesn't exist

Fixed #11160: Formset non_form_errors returns ErrorList() if is_valid is not called

Fixed #11915: generic Accept-Language matches country-specific variants

Added 'extra' argument for serialization of non-field attributes.