This removes another use of the gl::ErrorOrResult pattern. Unblocks
error refactoring.

Bug: angleproject:2738
Change-Id: Ib611a3b68f8995469befd51797dfed34eaeee84e
Reviewed-on: https://chromium-review.googlesource.com/1151450
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>