No.

You should not be trying to customize every single aspect of what Genio generates. That is not the best practice in model driven development. Genio is the automation of the development of the best solutions to each problem, and the aim is to continuously improve those solutions, not to allow increasingly more variants. Otherwise you end up with a standard programming language.

The validation field allows a message because the condition is a developer supplied expression. So the developer also has to supply a meaningful message to go along with it.

The unique rule is always the same and applies the same way, so using a standard message is basic good practice for usability. It establishes a coherent interface throughout the system for the end user. If the standard message can be improved, then it should be improved for everyone and every application.

Take into consideration if you have the Human Key correctly setup so that the 'unique' message can guide the user to the offending record with a meaningful Id for that record.