Werk #16736: Warn during update if contact groups are used in rulesets but not available

Some rulesets like 'Assignment of hosts (or services) to contact groups' use contact groups which may not be synced because they are assigned to a specific customer or to 'Provider' (default). This inconsistency might have already been present in your configuration but went unnoticed in the previous versions. With 2.3, we introduced new validation mechanics which now warn you about this potential issue.

With this change we make the shown error message a more helpful so that users get a hint how to solve the situation.

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