add a handler for unknown HTTP errors

This change adds logic to handle a situation where an error response has
been received by HTTP but its body schema is an unknown format.

This issue came up during a review of related changes:
https://review.opendev.org/#/c/662281/
https://review.opendev.org/#/c/662281/7/keystoneauth1/exceptions/http.py

Change-Id: I21a33052e951f515988fdfd8ab1f42440ca9d4f8

keystoneauth1/exceptions/http.py

@@ -436,6 +436,8 @@ def from_response(response, method, url):
                     kwargs["message"] = "{}{}".format(msg_hdr,
                                                       errors[0].get("title"))
                     kwargs["details"] = errors[0].get("detail")
+            else:
+                kwargs["message"] = "Unrecognized schema in response body."
 
     elif content_type.startswith("text/"):
         kwargs["details"] = response.text

keystoneauth1/tests/unit/test_session.py

@@ -641,6 +641,19 @@ class SessionTests(utils.TestCase):
             self.assertEqual(ex.message, msg)
             self.assertIsNone(ex.details)
 
+    def test_error_message_unknown_schema(self):
+        error_message = 'Uh oh, things went bad!'
+        payload = json.dumps(error_message)
+        self.stub_url('GET', status_code=9000, text=payload,
+                      headers={'Content-Type': 'application/json'})
+        session = client_session.Session()
+
+        msg = 'Unrecognized schema in response body. (HTTP 9000)'
+        try:
+            session.get(self.TEST_URL)
+        except exceptions.HttpError as ex:
+            self.assertEqual(ex.message, msg)
+
 
 class RedirectTests(utils.TestCase):
 

releasenotes/notes/improve-http-error-handling.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - Add logic to handle HTTP error responses that do not conform to a known
+    schema.