Prepare extension for nailgun exceptions refactoring

Change-Id: Ie9bc919aa1bb0c52c40d665c37356edda0c21b8c
Partial-Bug: #1566195

cluster_upgrade/__init__.py

@@ -0,0 +1,20 @@
+# coding: utf-8
+
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+try:
+    # old style errors
+    from nailgun.errors import errors  # NOQA
+except ImportError:
+    # new exceptions
+    from nailgun import errors  # NOQA

cluster_upgrade/tests/test_validators.py

@@ -19,10 +19,11 @@ import mock
 from oslo_serialization import jsonutils
 
 from nailgun import consts
-from nailgun.errors import errors
 from nailgun.settings import settings
 from nailgun.test import base
 
+from cluster_upgrade import errors
+
 from .. import validators
 from . import base as tests_base
 from . import EXTENSION

cluster_upgrade/validators.py

@@ -17,9 +17,10 @@
 from nailgun.api.v1.validators import assignment
 from nailgun.api.v1.validators import base
 from nailgun import consts
-from nailgun.errors import errors
 from nailgun import objects
 
+from cluster_upgrade import errors
+
 from .objects import adapters