Buffer: Minor error macro cleanups. Refactoring patch only. BUG=angleproject:1635 Change-Id: I9b86fc77f8f62a0f67f9fc73debdc8fb5e379bcc Reviewed-on: https://chromium-review.googlesource.com/450918 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>