title: OpenCV: fs.open fails
body: <p>I want to read from an .xml file, but this line</p>
  
  <pre><code>fs.open("test.xml", FileStorage::READ);
  </code></pre>
  
  <p>fails with cv::Exception.</p>
  
  <p>\opencv_source\opencv-3.4.1\modules\core\src\persistence_c.cpp:384: error: (-49) Input file is empty in function cvOpenFileStorage\n" ...} ...}    const cv::Exception &amp;</p>
  
  <p>I'm using Win7, OpenCV 3.4.1, c++, VisualStudio 2015. It's my fist time using .xml files and I'm new to opencv.</p>
  
  <p>Any ideas what's wrong?</p>