colexec: adjust the eager cancellation in parallel unordered sync a bit

[yuzefovich]

This commit adjusts the logic for swallowing errors in the parallel
unordered synchronizer because of the eager cancellation when the
synchronizer transitions into the draining state to swallow all errors
coming from an input if the input's context has been canceled by the
synchronizer.

Fixes: #69419.

Release note: None

Release justification: update to the new functionality.

[cockroach-teamcity]

This change is 

[yuzefovich]

I updated the commit to swallow all errors that occur after the synchronizer transitioned into draining and performed the eager cancellation of the input (as Andrei and I discussed).

[michae2]

This seems fine. In fact, it might even be fine ("fine" meaning necessary error information still propagates upward) under more complicated scenarios (writes, non-local flows) assuming that lower layers return the same error again for subsequent calls.

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @andreimatei)

[yuzefovich]

TFTR!

bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)