[MapProxy] cache missing tiles

[martin.koska3 at gmail.com]

Dear developers,

Is there a plan to include possibility like - "create missing tiles"?
It's similar as in ArcGIS server.

I found this possibility useful for instance: in the case if process of 
seeding is broken because
of no response from external WMS or because of other reasons.

Thank you,

Martin Koska
GIS specialist
SAZP
Slovakia