[resultdb] Reduce task sampling
[resultdb] Reduce task sampling

Task sampling is expensive. Right now, backend.runInvocationTasks throws
a list of task to process on first error. Instead, log the error
and continue to the next tasks.

R=332b6c56634d0f63001046c664a2370006a02420@chromium.org

Change-Id: I752c48064852a371a6499a0f69a7acbbba923130
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2052217
Auto-Submit: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Chan Li <332b6c56634d0f63001046c664a2370006a02420@chromium.org>
Reviewed-by: Chan Li <332b6c56634d0f63001046c664a2370006a02420@chromium.org>

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