[cv] catch panic in a worker and emit debug info.
[cv] catch panic in a worker and emit debug info.

R=qyearsley, yiwzhang

Bug: 1182446
Change-Id: I7118908ca1629a97d459c4f42535fa85163cbe4a
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2720363
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
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