session file cleanup may fail, e.g.,

Zend_Session_Exception: Zend_Session::start() - /PATH/libs/Zend/Session.php(Line:480): Error #8 session_start(): ps_files_cleanup_dir: opendir(/var/lib/php5) failed: Permission denied 


git-svn-id: http://dev.piwik.org/svn/trunk@2280 59fd770c-687e-43c8-a1e3-f5a4ff64c105