Problem: An error in a timer can make Vim unusable.
Solution: Don't set the error flag or exception from a timer. Stop a timer
if it causes an error 3 out of 3 times. Discard an exception
caused inside a timer.
Problem: Can't use input() in a timer callback. (Cosmin Popescu)
Solution: Reset vgetc_busy and set timer_busy. (Ozaki Kiichi, closes#1790,
closes#1129)
Problem: Crash when stop_timer() is called in a callback of a callback.
Vim hangs when the timer callback uses too much time.
Solution: Set tr_id to -1 when a timer is to be deleted. Don't keep calling
callbacks forever. (Ozaki Kiichi)
Problem: In a timer callback the timer itself can't be found or stopped.
(Thinca)
Solution: Do not remove the timer from the list, remember whether it was
freed.
