When a channel open fails, we use tx-abort instead of warning/error.
This means that the peer won't disconnect! And instead when a new
message arrives, we'll need to rebuild the dualopend subd (if missing).
Makes opens a bit easer to retry (no reconnect needed), as well as keeps
the connection alive for other channels we may have with that peer.
Changelog-Changed: Experimental-Dual-Fund: open failures don't disconnect, but instead fail the opening process