Exclude broken sources from leaflet-providers about xyzservices

[martinfleis]

Closed by #78

@benbovy Broken providers are now marked manually based on the CI result. Those unavailable for some reason now have TileProvider.status = "broken". Note that fully functional providers do not have a status attribute (I don't want to clutter the JSON).

[benbovy]

Additionally or alternatively, is there an easy way to quickly test during runtime the status of a provider and get the results, e.g., via Bunch.status and/or TileProvider.status properties?

I can imagine how this kind of information could be useful in downstream libraries for the UI (for example: in a selection widget grey out items for the basemaps that are not currently working). I'm not sure if it happens often that providers stop working temporarily, though.

[martinfleis]

@benbovy That is actually quite tricky to test because some providers restrict the access, so you can load it in leaflet but cannot download the tile itself (i.e. to be used in contextily). But I agree that it would be a nice thing to have.