fix: Do not exit if other songs can be downloaded

[grufkork]

When downloading tracks in parallell, the program exits as soon as the last track is finished whether that is an error or if it's actually finished. I believe this is the correct behaviour - a single track failing shouldn't prevent the rest from downloading. Regardless, in the previous implementation the "force exit" from errors was overridden regardless - only the very last song counted.

[grufkork]

I'm not sure what I wrote in my previous message - what I meant to say is that this fixes the case where if the last track finishes downloading before the second to last one, the program will exit and the second last song will be malformed and without metadata. The issue was further exacerbated when I was experimenting and had most tracks downloaded. When the last track "errored" as Already Downloaded, the whole program would exit. Rather, when one track fails, the program should just consider it finished and move on to the next task.