trusts: Make trust_show.get_dn raise properly formatted NotFound
The trust_show command does not raise a properly formatted NotFound
error if the trust is not found, only a generic EmptyResult error
is raised.
This patch makes the trust_show tell us what actually could not be
found.
https://fedorahosted.org/freeipa/ticket/5389
Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>