D3D11: Make createStagingTexture return Error. This removes the use of the ErrorOrResult pattern in Renderer11. Bug: angleproject:2738 Change-Id: Icc046ddd86394df56ca1acd10b1804fd6afa8ad0 Reviewed-on: https://chromium-review.googlesource.com/1142953 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>