Changed return status code from 404 to 403

Discussions on https://review.openstack.org/#/c/108516/10 led to
a consensus on the return status code to be a 403, however due
to crazy rebasing things it looks like that change was accidentally
reverted. This patch sets the HTTP return status code on DELETE
back to 403 for unauthorized users.

Change-Id: I0c10d78275a6f531067ec75e05bf1b814ed1d1a8
Merge "Changed return status code from 404 to 403"