title: qt use QNetworkAccessManager couldn't get the error signal
body: <pre><code>    m_request.setUrl(pDownloadInfo-&gt;url);
      m_preply = pManager-&gt;get(m_request);
  
      QObject::connect(m_preply, SIGNAL(error(QNetworkReply::NetworkError)),
              this, SLOT(DownloadError(QNetworkReply::NetworkError)));
      QObject::connect(m_preply, SIGNAL(finished()), this, SLOT(ReadyRead()));
  </code></pre>
  
  <p>I use qt5.0 and I use QNetworkAccessManager  to download the http file.
  I couldn't get the error signal when I input a wrong url which didn't correct,
  but I can get the finished signal when I input a correct url.</p>
  
  <p>What's reason about this question? </p>