Provide an error message if downgrading schema
Provide an error message if downgrading schema

The current error is not informative at all.

Change-Id: I2dd50324dab1c25f968f7914e278dbf492d05dc5

klmitch/keystone,dims/keystone,rajalokan/keystone,mahak/keystone,klmitch/keystone,rajalokan/keystone,ilay09/keystone,openstack/keystone,himanshu-setia/keystone,rajalokan/keystone,ilay09/keystone,himanshu-setia/keystone,openstack/keystone,openstack/keystone,cernops/keystone,dims/keystone,ilay09/keystone,mahak/keystone,mahak/keystone,cernops/keystone