Here's one way to do it using the Classic Designer.  This will do a pop-up alert on any validation errors, not just missing required fields.

$(document).ready(function () {

  //Alert User if required fields are missing.
  $(".action-btn.Submit").on('click', function() {
    var noErrors = true;
    $('#form1').parsley().validate();
    $('.parsley-error').each(function() {
      noErrors = false;
    });
    if (!noErrors) {
      alert('There are errors that must be resolved!');
    } 
  }); 

});

If you want it to work on any of the Submit/Approve/Reject/Save buttons instead of just Submit, then on line 4 change ".action-btn.Submit" to ".action-btn" instead.