Fixes wording of exceptions for delete
Fixes wording of exceptions for delete

Updated the wording of exceptions that prevent the deletion of
managed records. Original message indicated that user was
unable to update when the user actually requested a delete.
New message is "Managed records may not be deleted"
Both exceptions in designate/central/service.py

Change-Id: I27395999b2002374a74bddad71333f508354f37b
Closes-Bug: #1511230

grahamhayes/designate,openstack/designate,grahamhayes/designate,openstack/designate,openstack/designate,grahamhayes/designate