tv: delete failed broadcasts

As discussed in PR #271, broadcasts sometimes get created and then left in an upcoming state. I think we should delete the broadcasts if they fail to go live, or at least give the user that option.

@saxon-milton 's comment on the PR:
"there's the possibility that we create a broadcast but can't get the hardware properly started or get the broadcast to transition to a live state (even after retries), so we'd be left with an unused broadcast. I don't think we currently have any mechanism that removes broadcasts that don't eventuate (we could have this though). We could check for these sorts of conditions (especially unexpected conditions) and log accordingly."