detect when the connection with the server is closed when publishing with udp (#6)

examples/client-publish-tcp.go

@@ -59,6 +59,7 @@ func main() {
 		// write frames to the server
 		err = conn.WriteFrameTCP(track.Id, gortsplib.StreamTypeRtp, buf[:n])
 		if err != nil {
+			fmt.Println("connection is closed (%s)", err)
 			break
 		}
 	}