server.Shutdown() should print errors

[leventov]

zeroconf/server.go

Lines 195 to 198 in c2d1b41

	// Shutdown closes all udp connections and unregisters the service
	func (s *Server) Shutdown() {
	s.shutdown()
	}

Should be

if err := s.shutdown(); err != nil {
  log.Printf("Error shutting down the server: %v", err)
}

shutdown() itself should propagate or print errors from closing ipv4conn and ipv6conn.