Got a solution from http://sahi.co.in/w/sahi-call-back-functions
function onScriptError($e){
_focusWindow();
_takeScreenShot();
}
onScriptFailure = onScriptError;
_navigateTo("http://sahi.co.in/demo/training/");
_setValue(_textbox("user"), "test");
_setValue(_password("password"), "secret");
_click(_submit("Login"));