Glance can work in two different modes: split and single.
When File / NFS is configured w/ `type: split`, or if a different backend is configured w/ `type: single`, the webhook stops the update early enough, producing an "invalid layout error" like the following:

The OpenStackControlPlane "openstack-control-plane" is invalid: spec.glance.template.glanceAPIs[default]: Invalid value: "default": Invalid backend configuration detected 

If a human operator doesn't know enough about Glance and the reason behind this configuration, it might be confusing, and complicate the troubleshooting.
The idea behind this bug is to improve the operator to produce a better (static) message that clarifies the reason of the failure, suggesting the next step.