[CORS-RFC1918] Communicate errors as CORS errors.

This retires ERR_INSECURE_PRIVATE_NETWORK_REQUEST in favor of the same
scheme used by regular (non-RFC1918) CORS errors: ERR_FAILED along with
a CorsErrorStatus to further explain the error.

Fixed: chromium:1141851
Change-Id: Ie8356b63e5139c2bc0352f0e98dc17f90f287238
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2503510


Commit-Queue: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Auto-Submit: Titouan Rigoudy <titouan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823256}