modify_bg fails on windowless widget should produce gtk_warning

[george young <gry ll mit edu>]

[gtk+-2.4, pytgk-2.0.0]
When I do modify_bg on a Frame widget, nothing happens.  I understand 
(after hours of hacking) that this is because a Frame has no window.

It's fine that it fails, this is just an aspect of the toolkit, but 
is there some reason it can't emit a warning to stderr (or where ever
warnings are being emitted/logged), so the app developer doesn't waste
time tracking down a silent failure?  
It looks like there's a GTK_WIDGET_NO_WINDOW (widget) that could be 
easily tested...

[I assume this goes for set_style as well]

-- George Young

-- 
"Are the gods not just?"  "Oh no, child.
What would become of us if they were?" (CSL)