mod_s2s: Report outgoing connection failures to remote servers When setting up a new server, it can be easy to make mistakes with DNS, firewall or port forwarding that results in the ability to make outgoing connections, but not receive incoming, leading to a half-working setup where messages can be sent but not received. This can be difficult to diagnose. It is also difficult to inform the other end about the problem without a working channel in that direction, but when an incoming connection has been established, it can be closed with a stream error, which hopefully shows up in logs.