This refactor will allow us to generate different error types in
different backends. This makes Vulkan happy because it won't have to
generate gl::Errors and can stay with vk::Error.

Bug: angleproject:2713
Change-Id: I981402450f3b519d4f79851982547695d583355a
Reviewed-on: https://chromium-review.googlesource.com/1128921
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>