Timer::stop returns TIMER_NOT_AN_EVENT when timer was stopped

When Timer::stop returns TIMER_NOT_AN_EVENT when a timer was stopped it is more easy to invalidate a timer ID variable. 
  tmr = t.stop(tmr);  

Otherwise some side effect may happen especially when you use more than one timer, e.g.:
  t1 = t.every(200, foo);
  t.stop(t1);
  ....
  t2 = t.every(100, bar);
  t.stop(t1);  // for some reason you call stop(t1) here again 

may result in stopping t2 if t1 is not invalidated