internal: add HTTPError.Unwrap

This allows callers to access the underlying error via errors.Unwrap.

internal/internal.go

@@ -100,6 +100,10 @@ func (err *HTTPError) Error() string {
 	}
 }
 
+func (err *HTTPError) Unwrap() error {
+	return err.Err
+}
+
 // DAVError is a XML error with HTTP status and a human readable message
 type DAVError struct {
 	Code int