lxc/lxccontainer: stop printing misleading errors in enter_net_ns()

lxc/lxccontainer: stop printing misleading errors in enter_net_ns()

In enter_net_ns() we try to enter network namespace at first, before
entering a user namespace to support inherited netns case properly.
It is expected to get EPERM for unprivileged container with non-shared
network namespace at first try. Let's take this into account
and stop misleading users with these error messages.

Link: https://discuss.linuxcontainers.org/t/lxc-ls-fancy-command-shows-operation-not-permitted/24080
Fixes: 3011e79f92ef ("lxccontainer: fix enter_net_ns helper to work when netns is inherited")
Fixes: #4560
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>