[Flight] Don't call onError/onPostpone when halting and unify error branches (#31715)

We shouldn't call onError/onPostpone when we halt a stream because that
node didn't error yet. Its digest would also get lost.

We also have a lot of error branches now for thenables and streams. This
unifies them under erroredTask. I'm not yet unifying the cases that
don't allocate a task for the error when those are outlined.