trustdomain: Perform validation of the trust domain first
    
    Makes sure that the first check that is performed when trustdomain-del
    command is run is that the actual trusted domain exists. This is done to
    prevent a subseqent error which might be misleading.
    
    https://fedorahosted.org/freeipa/ticket/5389
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>