[cv] fix poller exception handling.
[cv] fix poller exception handling.

1. Transaction body can be retried.
1.1. But EVersion should be incremented only once.
1.2. Upon which a value that was in fact written from prior try may be read.
This should result in a transient error, not permanent one.
2. Simplify by annotating error with project name only in one place.

Also discovered by e2e tests with flaky datastore.

R=yiwzhang

Change-Id: Ib63e7568f3310d7ff20dc591a71b8cd950a83c60
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2803252
Commit-Queue: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

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