[MapProxy] Caching a TMS server returning missing or empty tiles

[Jean-Claude Repetto]

Hello,

I'm trying to use MapProxy to cache a TMS server. Sometimes, when the 
tile is empty, the server returns a http error 404 (Not found). Some TMS 
servers return a http error 204 (No content).

Example :
http://toolserver.org/tiles/bicycle/16/34333/22936.png -> OK
http://toolserver.org/tiles/bicycle/16/34333/22935.png -> 404

Is it possible to configure MapProxy so that missing (404) or empty 
(204) tiles are replaced by a default tile (generally a white tile, a 
black tile, or a transparent tile) ?

Thanks,
Jean-Claude