Fix: lttng UST and kernel consumer: fix ret vs errno mixup

Fix: lttng UST and kernel consumer: fix ret vs errno mixup

- errno should be set to -ret (not the opposite!)
  The main effect is that the perror errors were meaningless.

- errno should always be compared with positive values.
  This was also causing consumerd error handling mistakes.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>