Allow ListOnly request to time-out gracefully in web-ui when adding torrents

[scottopell]

This is split off from #285 (comment)

When I try to add a magnet link for an old / low-seed count torrent, the current Upload modal requires a ListOnly request to complete before proceeding to the next step (file selection) of the Add Torrent workflow.
Currently, this request prevents proceeding to the next step of adding the torrent via the web interface, so certain magnet links cannot be added.

To address this, the ListOnly request should have a graceful timeout and the File Selection step should be skipped, just selecting an output destination.

[scottopell]

Ah and I forgot that @ikatson also pointed out I'll need some changes in the backend to let the magnet resolution fail gracefully too

you need to update the backend to allow adding torrents without waiting for them to resolve the magnet.
#285 (comment)