The object specified does not belong to a list. at Microsoft.SharePoint.SPWeb.GetItem(String strUrl, Boolean bFile, Boolean cacheRowsetAndId)
at Microsoft.SharePoint.SPFolder.get_Item()
This seems to me a bug, but is documented in http://msdn.microsoft.com/en-us/library/ms461547.aspx
If the folder does not exist, an SPFolder object is returned, but any attempt to access its properties throws a FileNotFoundException exception.