I think there are meaningful things a beginner can do. I'd be surprised if improving error messages would be terribly hard. And working on the small issues will help you become familiar enough to help with the bigger things.
Edit: Also perhaps a way to get some exposure to the rust compiler is to help implement lints (in clippy) - it's basically a compiler plugin and relies on the same things available in the compiler.
The D-papercut diagnostics tickets are in my mind a great source of these kind of newcomers tasks, as some are quite small and don't require full context of the codebase.