Display an error if a song can't be fetched (e.g. if .zip is not found)

[n3tman]

Currently there's no indication if a song can't be found on Beatsaver website.

Message says "Downloading song...", example: https://supermedium.com/beatsaver-viewer/?id=811-533

Would be nice to implement a callback and change the message accordingly (like "Song can't be found").

[ngokevin]

Alright, working on it.

[ngokevin]

https://github.com/supermedium/beatsaver-viewer/commit/ca8245c7bc0251c859429a12f627f2b1d2013484