Images metadata cleanup

Adapt review comments on https://review.openstack.org/#/c/112874/
to admin/images/views.py regarding exception handling.

Change-Id: I31d96f37d1a4184f3df75a7ccb4a870477645307
Closes-Bug: 1370061
Co-Authored-By: Bartosz Fic <bartosz.fic@intel.com>
Co-Authored-By: Michal Dulko <michal.dulko@intel.com>
Merge "Images metadata cleanup"