Missing `/music` folder prevents engine from loading level

[paavohuhtala]

If the id1 folder does not contain a /music subfolder with music files (which is the case for most distributions of Quake 1), the engine does not load a level, and instead just shows the console. IMHO in case the music is missing the game should log a warning and move on.

The engine prints the following to stdout:

ERROR 2022-09-11T11:56:41Z: quake_client::game: Failed to load sound: No such music track: track02

This should not be a hard error, and from reading the code I don't understand why it prevents the level from loading.