There is only one such perror call, so remove it in nl.c

In this same patch, verify that all functions of nl.c and network.c
are reporting a -errno value in case of error;
value that is reported in lxc log by the callers in conf.c
Signed-off-by: Michel Normand <normand@fr.ibm.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>