Store users: Ensure the last owner cannot be downgraded (#6654)

* Store users: Ensure the last owner cannot be downgraded

Changes the behaviour of the `AddOrUpdateStoreUser` method to throw errors for the failure cases, so that the UI and API can report the actual problem. A role change might fail if the user already has that role or if they are the last owner of the store.

* Cleanup code

---------

Co-authored-by: nicolas.dorier <nicolas.dorier@gmail.com>

BTCPayServer/Controllers/GreenField/GreenfieldUsersController.cs

@@ -409,7 +409,6 @@ namespace BTCPayServer.Controllers.Greenfield
                         settings.FirstRun = false;
                         await _settingsRepository.UpdateSetting(settings);
                     }
-
                     await _settingsRepository.FirstAdminRegistered(policies, _options.UpdateUrl != null, _options.DisableRegistration, Logs);
                 }
             }