Catch error when run purchformletter

Hi All,

I want to catch error when convert purchase to packing slip:

dialog = purchformletter.dialog( );
    dialog.run() ;
    dialog.wait( );
    if (dialog.closedOk( ))
    {
        purchformletter.run() ;

I tried to put run method into a try/catch statement with no success. How can I catch if there are errors at run?