Werk #15525: Improve UI for edge cases in the service discovery page

A recent fix introduced a new message to the service discovery page in case no cached data was available for SNMP devices:

Could not find any service for this host. You might need to trigger a rescan.

This message was also shown if for some other reason the datasources failed, such as an unreachable host or a missconfiguration. In those cases a rescan would obviousy not help, and the user would just get this message again.

This is now mitigated by showing a different message once a rescan has been triggered:

No services found. If you expect this host to have (vanished) services, it probably means that one of the configured data sources is not operating as expected. Take a look at the Check_MK service to see what is wrong.

Note: The underlying issue here is, that the service discovery currently does not report on the state of the datasources, i.e., whether even the connection to hosts was successfull. If the connection can not be established, for instance because of changed SNMP credentials, users probably do not want to remove all of their services. While all services being vanished probably alerts an attentive user, the situation is even more intransparent if one of multiple datasources only creates few services. In such a case it is impossible to distinguish the truly vanished services from the ones only missing as a result of the failed datasource.

This more fundamental issue will be addressed in Checkmk versions 2.2 upwards.

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