refactor(fileService): improve error handling

[czaja307]

I really dislike how this fileService works, with it returning an error instead of just throwing.

Also the error should probably be rethrown, since it's a critical error anyway.

and of course it throws when I try to use it, looks like it doesn't try to create the storage dir if it doesn't exist

Originally posted by @mini-bomba in #87 (comment)