Returning an Error was costing us performance in extra
error checks. This intead uses the Context to process the
Error immediately, and returns a value from isComplete and
checkStatus.

Improves performance in draw call validation.

Bug: chromium:822235
Change-Id: I0793fc690e86137425fed593d45083e40aee8db9
Reviewed-on: https://chromium-review.googlesource.com/1011370Reviewed-by: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>