title: Use stderr in lua io.popen to determine faulty function call
body: <p>I'm making a function that can read the metadata of the current song playing in spotify. This is being programmed in lua since it is an implementation for awesome wm. I got the following line to get all the metadata that I can later use. </p>
  
  <pre><code>handle = io.popen('qdbus org.mpris.MediaPlayer2.spotify /org/mpris/MediaPlayer2 org.mpris.MediaPlayer2.Player.Metadata | awk -F: \'{$1=\"\";$2=\"\";print substr($0,4)}\'') 
  </code></pre>
  
  <p>However when Spotify is not running I don't get the expected information and qdbus writes an error to the stderr stream. I wanted to use the fact that qdbus writes to the error stream to determine a fault and stop the program there. (This should also catch any other errors not related to wheter spotify is running or not)</p>
  
  <p>My understanding is that lua popen uses popen3 that can subdivide between stdout and stderr. but all my efforts so far are fruitless and my error stream is always empty. Is it possible to check for a non nil value in the stderr in order to determine a faulty call to qdbus (or awk)? </p>
  
  <p>thanks!</p>