Memcached::get() always return NOT_FOUND on error

From https://github.com/php-memcached-dev/php-memcached/issues/9

    <?php

    $memcached = new Memcached();
    $memcached->addServer('localhost', 5555); // Server should not exist

    $result = $memcached->get('foo');
    echo $memcached->getResultMessage() . "\n";

Result: `NOT FOUND`
Expected: `SERVER IS MARKED DEAD`

Note: `$memcached->get('foo', null, $cas)` works.