Feedback after dialog is submited

when service dialog is submited, I need to validate user input and in case of bad values to stop and display error message to user (in web interface). I’m not sure how to stop processing stack of methods in one of the methods. For example just after CatalogItemInitialization I call custom method validate_dialog with simple exit call on error, but on service order it ends up always at requests page with error status.

Is there a way to display message to user in web interface, what was wrong as a simple feedback to submited dialog?