net: mdns_responder: Print return value if error in socket call

If socket call fails when mdns_responder creates listeners, then
print the return value to make it easier to figure out what is
wrong. Typically one needs to increase the size of
CONFIG_ZVFS_OPEN_MAX if errno is ENFILE.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>