Instead of messing with the script, I suggest that you set the error reporting not to display the Notice messages through php.ini
However, if you don't have access to edit the PHP configuration, then you can put this line on top of the functions.php file
PHP Code:
error_reporting(E_ALL ^ E_NOTICE);
For the Warning messages that are showing up, it's probably because the path to the session folder is incorrect, or it's not writable.
Again, you'll need to edit the php.ini, but if you can't try to use this code.
PHP Code:
// dir could be /home/user/session
// keep it OUT of webroot
ini_set("session.save_path", "/path/to/dir/with/chmod/777/perms");
Put the above code inside functions.php file before the session_start() is called.