Bundle an error message about whole buffer copies inside gl::Error
instead of delivering it to the Platform.

BUG=angleproject:1660

Change-Id: I13205b74529fbe40af05e7c1896c6315486e90e0
Reviewed-on: https://chromium-review.googlesource.com/437628
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>