UTIL: Add ERR_SBUS_REQUEST_HANDLED
    
    In most cases when sbus request parsing finishes, the request is handled
    internally and a reply is sent to the caller. However, in handlers that
    are parsed and handled completely manually, we might want to be notified
    about this case so that the called of sbus_request_parse_or_finish()
    aborts the request and doesn't proceed with using the sbus request which
    is already freed internally in sbus_request_parse_or_finish().
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>