Sometimes the generic "Please completed all the required fields" message isn't enough, and you want to describe in detail what the submitter should be doing with the field they didn't fill in or why you're requiring that information of them. You can do this with the $incompletefield tweak. Set up the field texts like this:

$incompletefield['links']['title'] = "You really do need to give a title for your listing.";
$incompletefield['links']['description'] = "You have to describe what your listing is about.";

Replace title and description with any field names. To find the field names, look at the input name attribute in the html or template or check the list at Admin Panel -> Customizations -> Add Fields.