Refactor Texture::copy*Image methods to use gl::Error objects. BUG=angle:520 Change-Id: I8b6bf5d50ee0d1f796eff52919ee65823fdee6cb Reviewed-on: https://chromium-review.googlesource.com/216647 Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>