Check the initialSongIndex before further processing the round

[fusion2004]

Right now, if you specify an invalid initialSongIndex, you don't get a notification until we download, transcode, generate announcers, play the stream intro and get ready to play the first track. That's a long while to wait and a lot of unnecessary work for a validation error.

We should move the check to as early as possible, just after fetching and reconciliation.