Fix: consumerd: type confusion in lttng_consumer_send_error

lttng_consumer_send_error sends an lttcomm_return_code to the session
daemon. However, the size of lttcomm_sessiond_command was used.

This was probably missed since the function accepts an integer instead
of a proper enum type.

The size accepted by the function is changed to use lttcomm_return_code
and the size of a fixed-size type is used to send the error code to the
session daemon.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I318e6a8d145373779d11557a70e43abca9783e5c