private String formatProcessingException(Exception e, String whileProcessing) {
        return String.format(
                    "Got exception of type %s with message '%s' while processing %s",
                    e.getClass().getName(),
                    e.getMessage(),
                    whileProcessing);
    }
Called from the constructor to ensure control and data handlers
are set and initialized.
documentapi/src/main/java/com/yahoo/documentapi/messagebus/MessageBusVisitorSession.java