_let_all_through logic

[CurleySamuel]

First we bucket exceptions and then we let only one of the exceptions actually perform the handling exception code to avoid redundancy.

In the current implementation using a semaphore I think there may be a race condition and it's possible we can handle the same exception multiple times. The code currently relies on the fact that the last person to request access to a semaphore will not be the next person to acquire it unless they're the only person.