At the moment, at least in my case (it can't be reproduced on the demo because of some limits) whenever a user goes to the domain icon on the top and doesn't have the 'custom dns service' enabled it appears an screen stating there's an excetion. A warning saying there's an error would be enough and more clear to the users.
QuoteAn error has been encountered
An exception with the following message has been thrown in file /var/www/imscp/gui/library/client-functions.php (Line: 334):
Feature custom_dns_record is not known by the customerHasFeature() function.