Exceptions should be more meaningful than what boto raises

In the openstack-dashboard apps we are rescuing from EC2ResponseError and checking the 'reason' field. If it is "Forbidden" we raise PermissionDenied, and otherwise raise NovaResponseError. We should generalize this to all adminclient requests so consumers of this library can have more explicit except clauses. See lp:openstack-dashboard file django-nova-syspanel/src/django-nova-syspanel/models.py method handle_nova_error for an example.