[buildbucket] Refactor the batch response error
[buildbucket] Refactor the batch response error

1. Properly handle grpc error in `toBatchResponseError()`, so that the
InvalidArgument error for large inputProperties can be correctly bubbled
up instead of being wrapped into the internal error.

2. Remove the `convertGRPCError()` which is a temporary hack for py+go
batch.

Bug: 1336469
Change-Id: I6f4123d809fec76aa1989ae492a08893708065ac
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3789736
Reviewed-by: Chan Li <332b6c56634d0f63001046c664a2370006a02420@chromium.org>
Commit-Queue: Yuanjun Huang <a5f4b8445ce5e958d19c0a7caeadcd07c9d18770@google.com>
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

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