C#下载网页（包含网页错误的情况）

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;
}