Force error when torrent data is moved from outside deluge

[s0undt3ch]

When a seeding torrent data is moved on the file-system from outside deluge, deluge starts thinking it's a new torrent and starts downloading it again(at least within server restarts). The correct behaviour would be to error the torrent giving it a meaningful status message like "Downloaded torrent data not found" or something like this, and if the user actually wants to re-download the torrent, have him resume it.