Customize error and HandleLoadErrors = True problem

I am trying to show a custom error instead of "Worklist item not found" in smartform.

I tried to set the ErrorPage directive of <%Page%>
or add a Page_Error handler to Page.Error event.

But both not working, the Smartform is so "smart" enough that still showing "Worklist item not found"

I found that in the smartform property, there is the property calls "HandlerLoadErrors", does anyone know what it mean? What is the different if I set it to True or False?