gerrit: log body if it can't be deserialized.
gerrit: log body if it can't be deserialized.

Currently, the error isn't sufficient to figure out what's wrong:
    code = Internal
    desc = failed to deserialize response: invalid character '<'
           looking for beginning of value

Also, changed internal error to panic. It's essentially an assertion
to assist developer adding/modifying on a Gerrit RPC.

R=qyearsley, yiwzhang

Bug: 1156241
Change-Id: Ia0c29f7688d50e9d870ddda6e5d2a774dea5d8df
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2604529
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Quinten Yearsley <747cf52e0cef2e3e9371802a2d344b6c87126a4b@chromium.org>
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

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