Feature: When the default layer visible on map fails, switch to another one

https://forum.camptocamp.org/t/carte-opentopomap-en-rade/285039

Though, I don't know if's possible to detect a massive fail. And as the user has a solution ()switch himself), the solution must be a very solid, straightforwrd one, not a hack.

Not the most urgent issue I would say, but if anybody is willing to have a look.