Allow a secondary/backup tileserver url

[thibaultmol]

Is your feature request related to a problem? Please describe.
It would be nice if you could specify a secondary url in case the primary tile server url doesn't work.

Describe the solution you'd like
have another parameter in the embed code to specify a second tile server if the first tileserver fails

Sorry if this is already in Openlayers, but I couldn't find it

[ahocevar]

You can specify a tileLoadFunction and handle error handling and fallback to a different server there. See https://openlayers.org/en/latest/apidoc/module-ol_Tile.html#~LoadFunction.

[thibaultmol]

Maybe it'd be good to have example of implementing a secondary tile server in the docs?

[ahocevar]

@thibaultmol If you want to create a pull request with such an example, it would be welcome!