Fix NameError that happens if _delete_thumbnails fails while updating a files location.
Fix NameError that happens if _delete_thumbnails fails while updating a files location.

Also provide a long explanation why update_location_on_storage overrides the default
transaction management offered by django's change_view.

pbs/django-filer,pbs/django-filer,pbs/django-filer,pbs/django-filer,pbs/django-filer