When config version parsing fails for both versions, the error message contains a reference to an error object. It references err, but it's always nil and the errors that actually contain useful details are oldErr and newErr. See: https://github.com/deepmap/oapi-codegen/blob/29ebe3262399dbbf9d9ce612757e50cd2ec2862f/cmd/oapi-codegen/oapi-codegen.go#L136-L161

Exposing those details to the user of the oapi-codegen cli would help with troubleshooting issues with the config file.

Example issues:

• error parsing configuration style as old version or new version: <nil> #831
• error when using config file #1034