Deleting pool with attached L7 policy sets LB to ERROR

[BenjaminLudwigSAP]

Steps to reproduce

1. Create a loadbalancer with a listener and a pool
2. On the listener, create a L7 policy (not extended L7 policy!) with action REDIRECT_TO_POOL, redirecting to the pool
3. Delete the pool, using the GUI

Expected behavior

Pool doesn't disappear from GUI, some kind of error message is shown, indicating that the L7 policy must be deleted first. The load balancer remains in state ACTIVE.

Observed behavior

Pool disappears from GUI, because pool entry in DB has provisioning_status DELETED, load balancer enters ERROR state.

Proposed fix

At pool delete requests, check whether there is a L7 policy connected to the pool and if it is, answer the request with an error.