code: func FileResult(l *State, err error, filename string) int {
  	if err == nil {
  		l.PushBoolean(true)
  		return 1
  	}
  	l.PushNil()
  	if filename != "" {
  		l.PushString(filename + ": " + err.Error())
  	} else {
  		l.PushString(err.Error())
  	}
  	l.PushInteger(0) // TODO map err to errno
  	return 3
  }
docs: // FileResult produces the return values for file-related functions in the standard
  // library (io.open, os.rename, file:seek, etc.).