trust-add: handle unavailable domain
    
    When ipa trust-add fails to retrieve the remote domain information,
    it exits with "an internal error has occurred".
    Handle the case with a better error message.
    
    Fixes: https://pagure.io/freeipa/issue/9488
    
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>