'retry' should mark child runs that weren't yet terminal as failed/canceled

[augray]

When a retry happens where one run is the parent of other runs, the child runs should be stopped if they were not already in a terminal state. If the retry was triggered by a failure in one of these child runs, and there was no parallelism, this will have happened naturally. However, there are two cases where a retry can be triggered where there are still active child runs:

(a) if there is parallelism in the child runs, and only one of the branches failed (other branches would still be active)
(b) if the parent had a timeout, and therefore the parent fails despite the children being otherwise healthy.

[augray]

This will be kind of tricky to do: currently there is an assumption that if there are any failed/canceled runs in the pipeline, the whole pipeline should be failed. Fixing this bug would mean changing that assumption.