title: QFile open file for writing fails
body: <p>I'm trying to open file and write some text data into it. </p>
  
  <pre><code>QFile out(":/test.txt");
  if (!out.open(QIODevice::ReadWrite)) {
      QMessageBox msgBox;
      msgBox.setText(out.errorString());
      msgBox.exec();
      return;
  }
  </code></pre>
  
  <p>But it fails with "Unknown error". 
  (Qt 4.6, Wnidows XP SP3)</p>