Yeah, your error handling should not always be backwards/forwards compatible. If the new version of a library is throwing a new kind of error, that library's clients need to be updated. Being able to break your clients when necessary is a feature & not a bug. Of course, if you don't need to break your clients, simply don't change your ErrorKind enum.

Backwards/forwards compatibility matters a lot across service boundaries. I'm not saying Protobuf needs to be exhaustively matchable. But native types from a static library? You want to be able to match those exhaustively.