The empty message field doesn't work as you expect. The empty field message will only be displayed if you enter data into the field - leave it, then come back and remove the data. Then your message will be displayed. Otherwise the validation message will be displayed.
You can write script on the validate event of the field and if you return true or false the message will appear but I think the generic message will always appear whether there is 1 field validation problem or more than 1.