src.ctf.lttng-live: make `lttng_live_get_one_metadata_packet()` return status

This is needed to differentiate real critical errors from relay daemon
from errors which are handled.

The relay daemon can return the LTTNG_VIEWER_METADATA_ERR status to
convey that the requested metadata stream is not found of the relay.
This can happen during normal operations if the associated trace is no
longer active. Short lived UST apps in per-pid session may produce such
scenario. In these cases, simply remove the live trace from the session
and go on with the iteration.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I452e63bd12a3c58d518726e3d178be241464bc2a
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2277