Add SerializerNotSupported error type to nailgun.errors

Now in case when type of task is not known to Nailgun proper error is
thrown.

Change-Id: I33923c04e3eba0ed8028efac51e36f347296d0ae
Closes-Bug: #1506915

nailgun/nailgun/errors/__init__.py

@@ -55,6 +55,8 @@ default_messages = {
     "ControllerInErrorState": ("One of the cluster controllers is in error "
                                "state, please, eliminate the problem prior "
                                "to proceeding further"),
+    "SerializerNotSupported": ("Serialization of the task is not supported "
+                               "because of unknown type"),
 
     # mongo errors
     "ExtMongoCheckerError": "Mongo nodes shouldn`t be used with external mongo",

nailgun/nailgun/orchestrator/tasks_serializer.py

@@ -426,7 +426,7 @@ class TaskSerializers(object):
             # deployment engine, therefore exception should be raised,
             # but it should be verified by validation as well
             raise errors.SerializerNotSupported(
-                'Serialization of type {0} not supported. Task {1}'.format(
+                'Serialization of type {0} is not supported. Task {1}'.format(
                     task['type'], task))
 
     def get_stage_serializer(self, task):

nailgun/nailgun/test/unit/test_stages_task_serialization.py

@@ -18,6 +18,7 @@ import mock
 import yaml
 
 from nailgun import consts
+from nailgun.errors import errors
 from nailgun import objects
 from nailgun.orchestrator.base_serializers import NetworkDeploymentSerializer
 from nailgun.orchestrator import deployment_graph
@@ -535,3 +536,20 @@ class TestConditionalTasksSerializers(BaseTaskSerializationTest):
 
         tasks = self.graph.pre_tasks_serialize(self.nodes)
         self.assertEqual(len(tasks), 0)
+
+
+class TestSerializationIsNotSupportedError(base.BaseTestCase):
+
+    def test_error_is_raised(self):
+        task_type = 'fake_type'
+        task = {'id': 'fake_task', 'type': task_type}
+
+        ts = tasks_serializer.TaskSerializers()
+
+        err_msg = 'Serialization of type {0} is not supported.'\
+            .format(task_type)
+
+        with self.assertRaises(errors.SerializerNotSupported) as exc:
+            ts.get_deploy_serializer(task)
+
+        self.assertIn(err_msg, exc.exception.message)