modcheck silently fails if you have an instance open since it can't delete old mods

[maskersss]

https://discord.com/channels/928728732376649768/1137930306557575198/1210513305286869042

[tildejustin]

this needs a lot of error handling in general, any file writing and web fetches should not be hard crashes, and even if they are, a helpful message should be shown. I want to do a full rewrite of logic (without ui change) for a lot of the ideas I have for outside parsing stuff and adding subsystems for that and that is basically the only time to make changes for this that won't just add clutter so this will have to wait for the summer.