If the previews folder isn't there an error occurs

@simong mentioned the interval should be higher as well.

```
[2013-03-08T09:10:00.236Z] ERROR: oae-cleaner/1628 on Berts-MacBook-Pro.local: Could not list the files.
    Error: ENOENT, readdir '/var/folders/rc/08k275f96gd93bqzv7yc1dhm0000gn/T//oae/previews'
    --
    directory: /var/folders/rc/08k275f96gd93bqzv7yc1dhm0000gn/T//oae/previews
```