UnityWebRequest.isNetworkError API 已经被弃用

`UnityWebRequest.isNetworkError` API已经被弃用，推荐使用 `UnityWebRequest.result == UnityWebRequest.Result.ConnectionError`代替，建议修改为

```cs
#if UNITY_2020_2_OR_NEWER
      if (m_WebRequest.result == UnityWebRequest.Result.ConnectionError)
#else
      if (m_WebRequest.isNetworkError)
#endif
```