handler_with_error.go
func HandlerWithError(handleFunc func(context.Context, *Conn, *Request) (result interface{}, err error)) *HandlerWithErrorConfigurer {
	return &HandlerWithErrorConfigurer{handleFunc: handleFunc}
}
func HandlerWithError(handleFunc func(context.Context, *Conn, *Request) (result interface{}, err error)) *HandlerWithErrorConfigurer {
	return &HandlerWithErrorConfigurer{handleFunc: handleFunc}
}
// HandlerWithError implements Handler by calling the func for each
// request and handling returned errors and results.