[core/theme] Handle errors during color loading

When loading a color scheme (e.g. wal) fails for some reason, do not
abort bumblebee-status. Instead, log an error message and continue
without the colors.

fixes #263

bumblebee/theme.py

@@ -160,8 +160,11 @@ class Theme(object):
 
     def _load_colors(self, name):
         """Load colors for a theme"""
+        try:
             if name == "wal":
                 return self._load_wal_colors()
+        except Exception as e:
+            logging.error("failed to load colors: {}".format(str(e)))
 
     def _load_icons(self, name):
         """Load icons for a theme"""