Extracted DB API layer

Extracted DB operations on Project and Story objects to a separate access
layer. That'll help to control SQLA operation more granullary.

This approach is used among most of OpenStack projects.

Change-Id: I0bde262eabb71c0613fcc06b93aefc70326269b7

storyboard/common/exception.py

@@ -0,0 +1,46 @@
+# Copyright (c) 2014 Mirantis Inc.
+#
+# 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.
+
+
+class StoryboardException(Exception):
+    """Base Exception for the project
+
+    To correctly use this class, inherit from it and define
+    the 'message' property.
+    """
+
+    message = "An unknown exception occurred"
+
+    def __str__(self):
+        return self.message
+
+    def __init__(self):
+        super(StoryboardException, self).__init__(self.message)
+
+
+class NotFound(StoryboardException):
+    message = "Object not found"
+
+    def __init__(self, message=None):
+        if message:
+            self.message = message
+
+
+class DuplicateEntry(StoryboardException):
+    message = "Database object already exists"
+
+    def __init__(self, message=None):
+        if message:
+            self.message = message