Description:
------------
When `sesssion.save_handler` is set to something unknown eg. 

- 'whatever'
- 'memcached' when memcached is not installed
- Value that reports `Warning: ini_set(): Session save handler "XXX" cannot be found for calling `ini_set('session.save_handler', 'XXX');

Then calling `session_set_save_handler()` has no effect.

Test script:
---------------
Set `sesssion.save_handler` to 'whatever' in `php.ini`


```
<?php

$initHandler = ini_get('session.save_handler');

session_set_save_handler(new MyHandlerImpl());

$setHandler = ini_get('session.save_handler');


echo 'Init Handler is: ' . $initHandler . PHP_EOL;
echo 'Set Handler is: ' . $setHandler . PHP_EOL;
```

Expected result:
----------------
Init Handler is: whatever
Set Handler is: user

Actual result:
--------------
Init Handler is: whatever
Set Handler is: whatever