Require explicit override for class when transparent error has an existing class

[dsherret]

It's too easy to accidentally override a transparent error's class: #2 (comment)

#[derive(Debug, JsError)]
#[class(type)]
pub struct FailedReadingLocalFileError {
  ...
}

#[derive(Debug, Error, JsError)]
pub enum FetchNoFollowErrorKind {
  #[class(generic)] // bad, this should have been `inherit`
  #[error(transparent)]
  ReadingFile(FailedReadingLocalFileError),