Any field with a red asterisk is supposed to signal the user that that field is mandatory, you probably knew this but just making sure, user education may be the quickest way to solve this.
Assuming you already know this here is what I would do. Hilary, the all wise and powerful, will probably have a better way but this is my stab at it.
For every field that is mandatory create another text box. So if the field ColorOfEyes and the field Gender are mandatory create two more textbox fields, say call it ColorEyeBlankCheck and GenderBlankCheck.
For each mandatory field set a rule, condition will be If ColorOfEyes is blank, set a fields value, this field will be the ColorEyeBlankcheck, and the value you set will be the text "Color Of Eyes Field cannot be blank". Do the same for the Gender field.
Create another textbox called SummaryOfBlankFields. The value of this field will be all the XBlankCheck field values concatonated together. If one of the mandatory fields has text in it nothing will be in the blankcheck value so you won't see it in the Summary field, but if the mandatory field is blank then the text will display.
Create a section, put the SummaryofBlankfields text box in there. Have text in the section that says Fields you have left blank: then place the Summary text box after it. Have a rule on the submit button that hides the section unless any of the mandatory fields are blank. This will be your "popup".
Option two:
Create a new view, the view educates the user by telling them some mandatory fields have been left blank and it has not been submitted, and to return to the form and look for the red asterisks. This view would be a rule on the submit button, the conditon would be if any of the mandatory fields are blank then switch views to this view. Have a return to the form button on the new view so the user newly educated can go back and fix their error then click submit again.