Add hints about how to return the correct type for functions that
return `Result<_, Box<dyn Error>`. Some feedback from Discord suggests
that people run into trouble with that.
Use `Box<dyn error::Error>` to allow solutions to use `?` to propagate
errors. In the tests, explicitly check `is_ok()` instead of trying to
force the error type to `String` (or other `PartialEq` type) using
`assert_eq!()`.
Taylor Yu