Portals4 BTL: Do not disqualify if a peer does not put Portals4 BTL modex info

If OPAL_MODEX_RECV() returns OPAL_ERR_NOT_FOUND, the peer didn't
send any Portals4 BTL info.  This is not a fatal error.  Instead of
disqualifying the Portals4 BTL just ignore that peer.

@jsquyres reported this in #194.