sbus: add sbus_request_reply_error()
    
    This simplifies error handling in sbus requests since we avoid
    creating DBusError and checking for NULL manually. It removes
    few lines of code.
    
    This patch does not replace all calls to sbus_request_fail_and_finish
    since sometimes it is desirable to create the error manualy. But
    it replaces it in most recent places.
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>