Update src/saml2/response.py

Made error message more descriptive

Co-authored-by: Ivan Kanakarakis <ivan.kanak@gmail.com>

Author: amit12297

src/saml2/response.py

@@ -791,7 +791,8 @@ def _assertion(self, assertion, verified=False):
                 try:
                     self.sec.check_signature(assertion, class_name(assertion), self.xmlstr)
                 except Exception as exc:
-                    logger.error("incorrectly_signed_response: %s", exc)
+                    logger.error("The signature on the assertion cannot be verified.")
+                    logger.debug("correctly_signed_response: %s", exc)
                     raise
 
         self.assertion = assertion