Failure notification is sent before "started" one when task fails early

[rshk]

Not a huge issue, but it might get confusing:

I guess it's caused by https://github.com/getsentry/freight/blob/master/freight/notifiers/utils.py#L55-L62 (why not something like: add to queue anyways -> if event == FINISHED then send all from queue, btw?)

[dcramer]

Dupe of #33

We could do the queue thing, but I'd prefer if we just have a way to revoke the pending notification (theres other situations we sort of need to as well, where the queue takes a minute and then it ends up sending all of them)