Fixed registration of private event listeners/subscribers

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6541
| License       | MIT
| Doc PR        | n/a

When registering a private event subscriber/listener, the listener was just ignored.

This PR does two things to fix the problem:

 * the event listener pass is done earlier to get access to the private services
 * when a private service is identified, a proper exception is thrown