Setting messages for mandatory fields

I have a ruleset in Incidents that sets mandatory fields.  Out of the box, when the field is left blank, the default msg that pops up to the user is "<field name> is required".

I know I can go into System Definition and change the caption for the field so it's a little more recognizable to the end user.  But how can I edit the msg altogether.  I've been ased to have a particular msg instead of saying "vendor is required." to say   “Please select the vendor that is responsible for the issue”

i know how to do this in format control but I have already done a lot of work in Process Deisgner around this ruleset, so does anyone know how to edit the default msg the process designer uses?