* Adds a nodiscard to the Error class.
* Fixes the places where Error was discard.
* Uses more vk::Error instead of gl::Error.

This enables the Vulkan back-end Error refactoring.

Bug: angleproject:2713
Change-Id: I09606d965e8b7cfd341ad7fb95cf029b9d694ef4
Reviewed-on: https://chromium-review.googlesource.com/1128922
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>