title: No error output when using subprocess with traceroute
body: <p>I'm trying to get the error message that is returned when a traceroute fails. For example: </p>
  
  <pre><code>from subprocess import CalledProcessError, check_output
  
  try: 
      output = check_output(["traceroute", "error"])
  except CalledProcessError as error:
      output = error.output
  
  print "error: {}".format(output)
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>error:
  </code></pre>
  
  <p>I've tried using <code>output = str(error.output)</code> but output stays empty. An error message is printed to the terminal when executing the above code, so it should be possible to assign it to a variable, right?</p>