upnp support just silently failed if upnp client is not available


git-svn-id: https://gnunet.org/svn/gnunet@33013 140774ce-b5e7-0310-ab8b-a85725594a96
handle nat_register fail


git-svn-id: https://gnunet.org/svn/gnunet@33014 140774ce-b5e7-0310-ab8b-a85725594a96