[limiter] Report the limit in logs and error replies.
[limiter] Report the limit in logs and error replies.

The error message now will be:
  limiter "rpc<=50": max concurrency limit

R=iannucci@chromium.org, 332b6c56634d0f63001046c664a2370006a02420@chromium.org

Change-Id: I670dc16b0e7fb55e54a77ad4f5e60ebaf5a9da0a
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2118611
Reviewed-by: Chan Li <332b6c56634d0f63001046c664a2370006a02420@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

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