Fix resend simulating & add trigger_missing_lineno
Fix resend simulating & add trigger_missing_lineno

  * we now don't send the triggered resend/comm errors out of band but
    instead as a response to the next received actual line - that
    prevents us from getting out of sync
  * new trigger_missing_lineno debug function to trigger a missing line
    number error _without_ a resend request for testing fix for #2226

Jaesin/OctoPrint,foosel/OctoPrint,Jaesin/OctoPrint,foosel/OctoPrint,foosel/OctoPrint,Jaesin/OctoPrint,Jaesin/OctoPrint,foosel/OctoPrint