Add a message to AttributeError in config.py

If you happen to try to use a non-existing config
variable inside a try-catch where you catch
Exception and then log its message, the resulting
error message isn't going to be very helpful when
the caught Exception itself has no message.

This adds a simple message to the AttributeError
which is raised by config.py if a non-existing
config variable is attempted to be used.

src/gpodder/config.py

@@ -179,7 +179,7 @@ class Config(dict):
             ( fieldtype, default ) = self.Settings[name]
             return self[name]
         else:
-            raise AttributeError
+            raise AttributeError('%s is not a setting' % name)
 
     def add_observer(self, callback):
         """