尽管存在内部错误，sessions_start仍返回true

Question

Following code:

    $s = @session_start();
    logMessage($this->debugLevel, ">>>>>>>>>>>>>>>>" . print_r($s, true));
    if ( $s === false )
    {
        // session not established!
        throw new \Exception("Session could not be started");
    }

I have configured memcached as session store. Memcache is not running and this generates a notice:

   PHP Notice:  session_start(): Server xxx (tcp 11211) failed with ...

so far as expected, BUT, $s is always true, even if the session obviously can not be started at all.

Q1: what's wrong with this? Q2: how can I catch the error ... ?

EDIT:

This:

    if ( session_start() === false )
    {
        // session not established!
        throw new \Exception("Session could not be started");
    }

doesn't change anything. Same as above. Exception NOT thrown.

EDIT 2:

memcache (1)

        $mc = new MemcachedConf();
        session_save_path('' . $mc );
        session_module_name("memcache");

memcache (2)

it's a 300 loc class, main part:

        if ( $this->memcache == null )
        $this->memcache = new Memcache();
        return $this->memcache;