title: Munin - how to return unknown value from plugin
body: <p>I want my munin plugin to report unknown values if the value can't be reported. What do I have to return? </p>
  
  <ul>
  <li>"nan"</li>
  <li>"-nan"</li>
  <li>nothing at all, just skip returning a value</li>
  <li>"unknown"</li>
  <li>...?</li>
  </ul>
  
  <p>Nothing works for me.</p>
  
  <p>Background: ntp_offset plugin fails if ntp is not running at all. I want to return unknown value and send an alert message. </p>