[boost] Boost::statechart: state reaction returned result

Could someone suggest me the recomended way of communicating error from fsm to the fsm user application? Assume state reaction unable to process event due to error and returns with discard_event(). As application calls process_event() to post an event which is of type "void". How do I make sure that my event was processed successfuly and not discarded? Thanks, Rajpal