How to modify Formated Message so it shows a more friendly m


I have a portlet that calls a webservice in IS and I’m trying to figure out how can I change the message displayed in the Formatted Message control to something more user friendly?
I think possibly it can be done with setValue from CAF.model(), but what do I need to do so that the portlet calls my CustomScript instead of rendering the exception message on page when the exception occours?