[Robustness test] Preserve return time for failed requests

Reference:
- https://github.com/etcd-io/etcd/issues/19579
- https://github.com/etcd-io/etcd/pull/19651#discussion_r2011549432
- https://github.com/etcd-io/etcd/pull/19669#discussion_r2013777795
- https://github.com/etcd-io/etcd/pull/19669#discussion_r2014694994

Signed-off-by: Chun-Hung Tseng <henrytseng@google.com>