Context

An unordered collection.BulkWrite that spans multiple batches can silently return success when an earlier batch's write-concern failure is overwritten by a later successful batch's nil WriteConcernError.

This is collection bulk-write, not client bulk-write. In triage, we assumed client bulk-write which prohibits propagating batch errors. Collection bulk-write does not have this prohibition. This is a legitimate bug.

Definition of done

The last non-nil write concern error should be propagated on BulkWriteException.