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

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.