Use case-scope error labels when handling LTTNG_CONSUMER_GET_CHANNEL

Use case-scope error labels when handling LTTNG_CONSUMER_GET_CHANNEL

This cleans-up the error handling of the case handling the
LTTNG_CONSUMER_GET_CHANNEL command. This is done in order to allow a
follow-up commit to perform some common clean-up operations on error
while introducing as few changes as possible.

No behaviour change is intended beyond additional calls to
health_code_update() in some code paths which should have no impact.

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