discovery: Don't start STUN/TURN disco on errorneous socket

If the initial attempt at sending discovery message returns a socket error, don't start the retransmit timer and immediately mark such discovery item as done. This is to quickly eliminate clearly non-functioning items from the discovery process.

Particularly improves times to finish discovery on Windows, where sending data from a link-local (169.254.0.0/16) IP to a destination not on the same subnet leads to "A socket operation was attempted to an unreachable network" error. Pointless retransmissions on those sockets prolonged discovery in the order of seconds.