When several torrents are being removed from session, an exception was
being raised in a callback function `on_status` with the `torrent_id` of
one (or more) of the removed torrents.
Now we will catch when the torrent does not exist anymore and print a
debug log about it.
Closes: https://dev.deluge-torrent.org/ticket/3498
Closes: https://github.com/deluge-torrent/deluge/pull/341