Print a useful message for unknown server errors

If a server error is returned that has an unknown code, novaclient will
end up printing something similar to:

ClientException: <attribute 'message' of 'exceptions.BaseException' objects>

Setting a message for ClientException will ensure that something more
useful than that is printed.

Change-Id: I43a2a33017f9a5c1b79d7fd8af4153e91d296f7b
Closes-bug: 1295293

novaclient/exceptions.py

@@ -81,6 +81,8 @@ class ClientException(Exception):
     """
     The base exception class for all exceptions this library raises.
     """
+    message = 'Unknown Error'
+
     def __init__(self, code, message=None, details=None, request_id=None,
                  url=None, method=None):
         self.code = code

novaclient/tests/test_http.py

@@ -13,6 +13,7 @@
 
 import mock
 import requests
+import six
 
 from novaclient import client
 from novaclient import exceptions
@@ -37,6 +38,12 @@ bad_req_response = utils.TestResponse({
 })
 bad_req_mock_request = mock.Mock(return_value=(bad_req_response))
 
+unknown_error_response = utils.TestResponse({
+    "status_code": 503,
+    "text": '',
+})
+unknown_error_mock_request = mock.Mock(return_value=unknown_error_response)
+
 
 def get_client():
     cl = client.HTTPClient("username", "password",
@@ -133,3 +140,16 @@ class ClientTest(utils.TestCase):
         cl2 = client.HTTPClient("username", "password", "project_id",
                                 "auth_test", http_log_debug=True)
         self.assertEqual(len(cl2._logger.handlers), 1)
+
+    @mock.patch.object(requests.Session, 'request', unknown_error_mock_request)
+    def test_unknown_server_error(self):
+        cl = get_client()
+        # This would be cleaner with the context manager version of
+        # assertRaises or assertRaisesRegexp, but both only appeared in
+        # Python 2.7 and testtools doesn't match that implementation yet
+        try:
+            cl.get('/hi')
+        except exceptions.ClientException as exc:
+            self.assertIn('Unknown Error', six.text_type(exc))
+        else:
+            self.fail('Expected exceptions.ClientException')