Use real exceptions for network create and destroy
Use real exceptions for network create and destroy

In order to provide reasonable error messages to the user, we
need real exceptions instead of a bunch of random value Errors.

Partially-implements blueprint better-support-for-multiple-networks

Change-Id: Id725afbdde40cba879608d15de9aa2753a256d10

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects