[1.6.x] Fixed #20791 -- Reworded ForeignKey default error message
[1.6.x] Fixed #20791 -- Reworded ForeignKey default error message

Backport of 311c1d2848 from master.

redhat-openstack/django,felixjimenez/django,django-nonrel/django,redhat-openstack/django,dex4er/django,django-nonrel/django,felixjimenez/django,felixjimenez/django,django-nonrel/django,dex4er/django,felixjimenez/django,dex4er/django,redhat-openstack/django,django-nonrel/django,redhat-openstack/django