Peoplecode Question about Page.Field custom error message

General page field/error question and apologies for posting to this forum but it was impossible for me to find where I should really ask it :)

I have a page field which has a prompt table behind it (prompt table with edit property). It can't be changed to "no edit". I created a custom error message to occur based on an edit but my custom message is superseded by the standard system message (15,11 Invalid value). I was wondering how to get my custom message to pop. I have the code in field edit and save edit but the system message takes precedence so my code doesn't get executed. Perhaps this is not even possible, I just need to know so