stout on no backup

Checks if the len(mbsacct.backupUDID) is > 0, it would print an error message instead of:

Traceback (most recent call last):
  File "./iloot.py", line 538, in <module>
    download_backup(args.apple_id, args.password, args.output, args.item_types, args.combined, args.itunes_style)
  File "./iloot.py", line 505, in download_backup
    if i == 0:
UnboundLocalError: local variable 'i' referenced before assignment