Updated Renderer::sync to return Error objects.

BUG=angle:520

Change-Id: I8f1f095977bab3848b9215cda1127a71982858b8
Reviewed-on: https://chromium-review.googlesource.com/220800
Reviewed-by: Nicolas Capens <capn@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>