[cv] handle both cases of spurious HTTP 404 when posting start msg.
[cv] handle both cases of spurious HTTP 404 when posting start msg.

Posting start message may do 2 Gerrit RPCs:
  * GetChangeInfo
  * SetReview

If outdated replica is hit, HTTP 404 may be returned for either.
Previously fix was only for SetReview.

Also, reduce annotation boilerplate, similar to
https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3226267

R=ddoman, yiwzhang

Bug: 1259964
Change-Id: I14ac7667185f9042fecffc1f4ea0073c5a85561c
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3228214
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Reviewed-by: Scott Lee <1dcb292530f82960316cb3d87cd67c3f212b79cb@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go