You can use the On-Error trigger in oracle forms to override the default functionality. To be able to do taht your first need to check what is the FRM error id of the error message. Then you can write something like this:
IF error_code = <error id> THEN
Message('Your Custom Error Message');
END IF;
RAISE FORM_TRIGGER_FAILURE;
It is important that you remember the raise form_trigger_failure because otherwise the program flow will continue as the error never have happened. This can of course be used for suppressing unwanted error messages but that is another story.