gitiles: map more http status code to corresponding grpc code
gitiles: map more http status code to corresponding grpc code

R=gbeaty

Bug: 1322939
Change-Id: If2ad7b4c6afe9be87fbaa5ddfde4a0b03e1a2b7d
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3631098
Auto-Submit: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Commit-Queue: Garrett Beaty <a0767768af30bcbe01b6de5024355c539362a830@google.com>
Reviewed-by: Garrett Beaty <a0767768af30bcbe01b6de5024355c539362a830@google.com>

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