manager: avoid infinite loop for unexpected waitid() error (#8168)

I think if we log the error as being _ignored_, we should also consider
the event as handled and clear it.  This was the behaviour prior to
575b300b (PR #7968).

I don't think we particularly wanted to change behaviour and keep retrying.
Sometimes that's useful, other times you cause more problems by filling the
logs.

Plus a nearby typo fix.