Vulkan: Clean up Error usage. * 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>