Getmore tells out real reason when collection was dropped due to rename

Hi,

Now a getMore request will tells out the real reason that the collection was dropped during getMore calls. But if the collection was dropped due to a renameCollection with dropTarget option was set, it will only return out a cursor not found error. Can we also tell out this reason so that user can know what really happens. The attached patch can simply do this, pls have a look. Thanks!