Not avaible data provider blocks select data provider dialog

[EHJ-52n]

The select data provider dialog is blocked (not usable at all) when one of the configured data providers is not available (e.g. long response time or some 5xxer error).

From my point of view this should result in an error message that this one provider is not available at the moment, giving the error message, in the data provider list. This would enable users to communicate the service error to a responsible service maintainer. The other available providers should be displayed as usual.

We might extend the API response to show service contact information if available in the SOS capabilities, or via manual entry in the settings.json.