[batch] fix throttler bugs (#6959)
[batch] fix throttler bugs (#6959)

I thought first completed would also fire on first exception,
but it appears not. These futures should only finish by raising
an exception anyway, so `FIRST_EXCEPTION` is fine.

I failed to realize `pending` was a `set` and thus adding an element
is called `add` not `append`.
cseed/hail,danking/hail,hail-is/hail,cseed/hail,hail-is/hail,danking/hail,hail-is/hail,danking/hail,danking/hail,danking/hail,hail-is/hail,danking/hail,hail-is/hail,cseed/hail,cseed/hail,cseed/hail,cseed/hail,danking/hail,cseed/hail,cseed/hail,hail-is/hail,hail-is/hail,hail-is/hail,danking/hail