rpcd-mod-lxc: info: return NOT_FOUND if container doesn't exist

rpcd-mod-lxc: info: return NOT_FOUND if container doesn't exist

It allow distinction between not existing and stopped container. So far
querying for not existing container was resulting in:
> ubus call lxc info '{ "name": "foo" }'
{
        "name": "foo",
        "state": "STOPPED",
        "ips": [

        ]
}

Now it's an error and it matches lxc-info command line:

> ubus call lxc info '{ "name": "foo" }'
Command failed: Not found

> lxc-info --name foo
foo doesn't exist

Signed-off-by: Rafał Miłecki <redacted>