[gerrit] Fix minor issues in Gerrit API.
[gerrit] Fix minor issues in Gerrit API.

* Use a/ prefix for all requests to use OAuth always.
* Handle status codes >200 and <300 correctly.

Change-Id: Iac1225f080fea66b4eaaee2c54dbdf10780c6a7e
Reviewed-on: https://chromium-review.googlesource.com/811839
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Reviewed-by: Roberto Carrillo <b31161e510fbb37f8f70fdcde11530d2bae88470@chromium.org>
Commit-Queue: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>

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