Fix: consumer: rotation error return codes

The return codes of rotation commands from consumerd are not
in sync with reality. Some are simply copy-pasted from old code.

Add new return codes to describe each error situation, and split the
"channel lookup" error from other errors so sessiond can distinguish
between an error caused by an exiting application (per-pid buffers)
(LTTCOMM_CONSUMERD_CHAN_NOT_FOUND) and an actual error while performing
the command.

Move the channel lookup into the rotation caller, because it is used by
two sub-functions: we don't want to lookup to succeed for the first and
then fail for the second.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>