Use "?:" as the operator string to make the error message clearer.

Update some comments to refer to specific revisions of the spec, since
there have been some spec changes in this area.

BUG=angleproject:2032

Change-Id: Ic5d128391bfa46187ad335228077f5eab9eac349
Reviewed-on: https://chromium-review.googlesource.com/500270Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>