@Nullable abstract fun handleErrorMessageToClient(errorMessage: Message<P>): Message<P>
Handle errors sent from the server side to clients, e.g. errors from the org.springframework.messaging.simp.stomp.StompBrokerRelayMessageHandler because connectivity failed or the external broker sent an error message, etc.
errorMessage - the error message, never null
Return
the error message to send to the client, or null in which case no message will be sent.