WebTorrent 0.91 changed how duplicate torrents are handled, which broke
handling in WebTorrent Desktop.
After this PR:
- No more try-catch on client.add -- this has never thrown errors.
- No check for duplicate torrent.key value since client.add no longer
returns the same torrent object when adding a duplicate torrent. It
emits 'error' instead, and that case is already handled :)
9eeb8133af