Hello,
If you are using the Form controller, you can use the obligatoryErrorGenerator, obligatoryCheckboxErrorGenerator and obligatoryRadioErrorGenerator parameters to return custom error messages (see the Ibex documentation). I think you can find an example of how to use it in the script of newly created experiments on the original Ibex Farm.
If you are using the PennController element Html, you can use the commands checkboxWarning, inputWarning and radioWarning.
Best,
Jeremy