Removing non existent route (both ipv4 and ipv6) using
rtnl_route_delete return NLE_FAILURE generic error code. After
investigation, it seems the linux kernel return -ESRCH in that case
(see http://lxr.linux.no/linux+v2.6.38/net/ipv6/route.c#L1367). Same
behaviour apply for removing non existent address. The function
nl_syserr2nlerr (lib/error.c) which is responsible for translating
from kernel error to libnl error doesn't handle ESRCH.