C#下载网页，即使网页404或者500错误

public static string GetWebPageAsString(string url)  {      HttpWebRequest httpWebRequest = (HttpWebRequest) WebRequest.Create(url);      HttpWebResponse httpWebResponse = null;      string xml = "";      try      {          httpWebResponse = (HttpWebResponse) httpWebRequest.GetResponse();      }      catch (WebException exception)      {          if (exception.Status == WebExceptionStatus.ProtocolError)          { //get the response object from the WebException              httpWebResponse = exception.Response as HttpWebResponse;              if (httpWebResponse == null){ return "<Error />";}          }      }      Stream stream = httpWebResponse.GetResponseStream();      StreamReader streamReader = new StreamReader(stream, Encoding.ASCII);      xml = streamReader.ReadToEnd();      //streamReader.Close();      if (httpWebResponse.StatusCode != System.Net.HttpStatusCode.OK)      {          throw new Exception(xml);      }         return xml;  }