Currently it is not possible to return a customized HTML error page if an error occurs in the Authenticate function ([L70](https://github.com/d-velop/dvelop-sdk-go/blob/0c1d447d56966de88e119289758c6d8d6dc6a5dc/idp/authmiddleware.go#L70), [L85](https://github.com/d-velop/dvelop-sdk-go/blob/0c1d447d56966de88e119289758c6d8d6dc6a5dc/idp/authmiddleware.go#L85), [L91](https://github.com/d-velop/dvelop-sdk-go/blob/0c1d447d56966de88e119289758c6d8d6dc6a5dc/idp/authmiddleware.go#L91), [L97](https://github.com/d-velop/dvelop-sdk-go/blob/0c1d447d56966de88e119289758c6d8d6dc6a5dc/idp/authmiddleware.go#L97), [L111](https://github.com/d-velop/dvelop-sdk-go/blob/0c1d447d56966de88e119289758c6d8d6dc6a5dc/idp/authmiddleware.go#L111)). It would be nice to have a custom render error page function.
