Ensure that failing ipts are eventually marked Faulty

Working on #1234 (closed), I've found that we can't always tell whether a failure is an intro point's fault, so I am making is_ipt_failure() only return true when we can be certain that the intro point is to blame. This behavior is fine so long as we eventually stop using an intro point if it fails for many times in a row.

If this is the current behavior, that's great, and there's nothing to do in the code.