cur_frm.cscript.custom_validate = function(doc) {
  $.each(wn.model.get("Sales Order Item", {parent:doc.name}), function(i, d) {
    if(d.amount <= 0) {
      var msg = "Please enter a value for X for item " + d.item_code;
      msgprint(msg);
      throw msg;
      // or use wn.throw(msg) to do both msgprint and throw in one go, if it is available in your version
    }
  }
}