Fix: stop lttng-relayd threads on health thread error

The lttng-relayd health thread may fail to initialize for
a variety of reason (notably, a too long unix domain socket
address), which will cause it to never notify that it is
ready.

In such circumstances, the lttng-relayd command, in background or
daemonize mode, will never return as the daemon's "readyness"
will never be signaled.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>