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

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.
Fixed #18990 -- Loaddata now complains if fixture 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.
fix warnings imports in fixtures tests
Merge pull request #1134 from senko/ticket_18990

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