[core/popup_v2] Add logging if tkinter is not found

bumblebee/popup_v2.py

@@ -1,5 +1,7 @@
 """Pop-up menus."""
 
+import logging
+
 try:
     import Tkinter as tk
 except ImportError:
@@ -7,7 +9,7 @@ except ImportError:
     try:
         import tkinter as tk
     except ImportError:
-        pass
+        logging.warning("failed to import tkinter - bumblebee popups won't work!")
 
 import functools