Improve STUN resolution error message

Add network to resolve stun host error message
Most error message in gatherCandidatesSrflx and
gatherCandidatesSrflxUDPMux include the network in
them but not the one when failing to Resolve the host.

gather.go

@@ -400,7 +400,7 @@ func (a *Agent) gatherCandidatesSrflxUDPMux(ctx context.Context, urls []*stun.UR
  hostPort := fmt.Sprintf("%s:%d", url.Host, url.Port)
  serverAddr, err := a.net.ResolveUDPAddr(network, hostPort)
  if err != nil {
- a.log.Debugf("Failed to resolve STUN host: %s: %v", hostPort, err)
+ a.log.Debugf("Failed to resolve STUN host: %s %s: %v", network, hostPort, err)
  return
  }
 
@@ -462,7 +462,7 @@ func (a *Agent) gatherCandidatesSrflx(ctx context.Context, urls []*stun.URI, net
  hostPort := fmt.Sprintf("%s:%d", url.Host, url.Port)
  serverAddr, err := a.net.ResolveUDPAddr(network, hostPort)
  if err != nil {
- a.log.Debugf("Failed to resolve STUN host: %s: %v", hostPort, err)
+ a.log.Debugf("Failed to resolve STUN host: %s %s: %v", network, hostPort, err)
  return
  }