Gerrit sometimes returns a full response json object at
the same time as returning a non-200 status code. This
refactor makes it easier for calling code to request
access to that object and handle error cases on its own.

The original version of this commit had a bug where
ReadHttpResponse properly set the default value for
accept_statuses, but all calls which came through
ReadHttpJsonResponse were setting None instead.

Bug: 710028
Change-Id: I8cee435d8acd487fb777b3fd69b5e48e19d2e5a3
Reviewed-on: https://chromium-review.googlesource.com/481060Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>