Handle errors in HTTP 200 reply when posting a comment
There are cases where the backend replies with HTTP status 200 after submitting a comment (will be changed in https://gitlab.com/gitlab-org/gitlab-ce/issues/61375). On the frontend we inspect the response in some cases to treat it as an error (https://gitlab.com/gitlab-org/gitlab-ce/blob/v11.10.0/app/assets/javascripts/notes/stores/actions.js#L284-286). We should always do that.
This will be a quick fix for https://gitlab.com/gitlab-org/gitlab-ee/issues/11445.