Add custom error types instead of using `String` as the error type about cafebabe

[staktrace]

Yeah, that's fair. I got a bit lazy with that when I wrote the code since I didn't need strong error typing. Will one error type suffice for you, or would it be useful to have different error types for different kinds of errors? Maybe an enumeration error type? Let me know.

I also need to audit the way I glue together the error strings, I think I changed my approach partway through so some of the error strings might not make sense.

[staktrace]

FYI I'm actively working on fixing this.

[staktrace]

I pushed commits to main that add a ParseError type

[staktrace]

This is all done now.