filesize() failure in email.php

Can happen when the backup is over `2GB`. Should be suppressed.

````
filesize(): stat failed for /home/content/00/5308500/html/wp-content/backupwordpress-cecadf6ecf-backups/americanshredding-com-default-1-file-2014-01-23-11-15-35.zip, /home/content/00/5308500/html/wp-content/plugins/backupwordpress/classes/email.php, 168 
````