handle util.popup ImportError gracefully

* util.popup requires tkinter to be installed (in order to display
  popups). As most people will probably use the default configuration
  of the pulseaudio module (where the popup is disabled) and in order
  to avoid breaking existing setups, we catch import errors and just log
  them.

bumblebee_status/modules/core/pulseaudio.py

@@ -43,8 +43,11 @@ import core.input
 import util.cli
 import util.graph
 import util.format
-import util.popup
 
+try:
+    import util.popup
+except ImportError as e:
+    logging.warning("Couldn't import util.popup: %s. Popups won't work!", e)
 
 class Module(core.module.Module):
     def __init__(self, config, theme, channel):