fix bug: Unwrap for withStack should first try to unwrap withMessage

errors.go

@@ -160,7 +160,13 @@ type withStack struct {
 func (w *withStack) Cause() error { return w.error }
 
 // Unwrap provides compatibility for Go 1.13 error chains.
-func (w *withStack) Unwrap() error { return w.error }
+func (w *withStack) Unwrap() error {
+	if e, ok := w.error.(interface{ Unwrap() error }); ok {
+		return e.Unwrap()
+	}
+
+	return w.error
+}
 
 func (w *withStack) Format(s fmt.State, verb rune) {
 	switch verb {