Glance GET /v2/images fails with 500 due to erroneous policy check

This patch will fix below two issues of V2 ResponseSerializer for
images-list, image-show, image-update and image-download.

1. A user should be able to list/show/update/download image without
needing permission on get_image_location.
2. A policy failure should result in a 403 return code. We're
getting a 500.

Fixes bug 1231255

Change-Id: Ie0ec2d574eea4433c4f610ec66a22cb16cae6dc6
Merge "Glance GET /v2/images fails with 500 due to erroneous policy check" into milestone-proposed