Enhance invalid document format info with reason

Signed-off-by: Ivan Kanakarakis <ivan.kanak@gmail.com>

Author: c00kiemon5ter

src/saml2/sigver.py

@@ -1496,6 +1496,7 @@ def _check_signature(self, decoded_xml, item, node_name=NODE_NAME, origdoc=None,
         except XMLSchemaError as e:
             error_context = {
                 "message": "Signature verification failed. Invalid document format.",
+                "reason": str(e),
                 "ID": item.id,
                 "issuer": _issuer,
                 "type": node_name,