More gracefully handle an exception raised during playback

[anoadragon453]

Currently if a song fails midway through playback, we'll just log an exception and carry on to the next song.

https://github.com/anoadragon453/busty/blob/012b6d257e722e6487ad727a25208b8772a3eb02/main.py#L556-L558

I think we could handle this in a slightly better way. Namely:

• Try replaying the song (though be careful of falling into an infinite loop).
• Send a message in the room that playback failed, rather than having no feedback on Discord. Otherwise everyone but the author will just think that the song simply ended abruptly.

[Cephian]

If there's a failure partway through I'm more inclined to believe it's an issue with the file than with some external factor which could be fixed by restarting playback. Sending a message in the room sounds like a good idea.