Also returning a gl::Error everywhere.

Bug: angleproject:2478
Change-Id: Ic8cae0ee7aee318bb95b3588044c34c62707b578
Reviewed-on: https://chromium-review.googlesource.com/1020083Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Luc Ferron <lucferron@chromium.org>