Fix: implicit conversion of enum types in consumer

This actually remove the use of LTTNG_ERR* code in the sessiond/consumer
protocol since it should NOT be used and some comparison of enum
(lttng_error_code vs lttcomm_return_code) were broken and dangerous.

Signed-off-by: David Goulet <dgoulet@efficios.com>
Conflicts:
	src/bin/lttng-sessiond/ust-consumer.c
	src/common/kernel-consumer/kernel-consumer.c