Team and project groups delete methods

Added delete methods for teams and project groups.
Now delete methods check project groups and teams for empty status.
DB api now raises exceptions if operation is no allowed.

Change-Id: Ic34d42d07c72683aa9624f45dcdf4c033af74e28

storyboard/common/exception.py

@@ -46,3 +46,11 @@ class DuplicateEntry(StoryboardException):
     def __init__(self, message=None):
         if message:
             self.message = message
+
+
+class NotEmpty(StoryboardException):
+    message = _("Database object must be empty")
+
+    def __init__(self, message=None):
+        if message:
+            self.message = message