fix: change 'internal_failure' condition to also use `error' field (#387)

* fix: change `internal_failure` condition to also use `error' field

In some cases response doesn't contain `error_description` field. So the retry won't work. The solution is to look at both `error` and `error_description` fields.
diff --git a/tests/oauth2/test__client.py b/tests/oauth2/test__client.py
index c415a1f..9cf59eb 100644
--- a/tests/oauth2/test__client.py
+++ b/tests/oauth2/test__client.py
@@ -112,15 +112,21 @@
 
 def test__token_endpoint_request_internal_failure_error():
     request = make_request(
-        {"error": "internal_failure", "error_description": "internal_failure"},
-        status=http_client.BAD_REQUEST,
+        {"error_description": "internal_failure"}, status=http_client.BAD_REQUEST
     )
 
     with pytest.raises(exceptions.RefreshError):
         _client._token_endpoint_request(
-            request,
-            "http://example.com",
-            {"error": "internal_failure", "error_description": "internal_failure"},
+            request, "http://example.com", {"error_description": "internal_failure"}
+        )
+
+    request = make_request(
+        {"error": "internal_failure"}, status=http_client.BAD_REQUEST
+    )
+
+    with pytest.raises(exceptions.RefreshError):
+        _client._token_endpoint_request(
+            request, "http://example.com", {"error": "internal_failure"}
         )