Show error when preventing groups with subscriptions being deleted

What does this MR do and why?

Related to #461495 (closed)

The grouptenant scale team is building out the Organization MVC. For more information about what an "Organization" will be see https://docs.gitlab.com/ee/architecture/blueprints/organization/

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Start GDK in SaaS mode
   • export GITLAB_SIMULATE_SAAS=1 gdk restart
2. Enable the feature flag
   • echo "Feature.enable(:ui_for_organizations)" | gdk rails c
3. Navigate to /-/organizations/default/groups_and_projects?display=groups
4. Click New group and fill out the form
5. Go to /admin/groups
6. Find the group you just created and click Edit
7. In the Permissions and group features section change the Plan dropdown to Ultimate
8. Navigate to /-/organizations/default/groups_and_projects?display=groups
9. Try to delete the group