Werk #11617: Activate changes: Improve error handling when trying to activate changes of offline sites

When triggering the activate changes API calls in "dirty" mode (activate all changes on all dirty sites), the offline sites were automatically excluded. In this situation the activation was performed with an empty list of sites. Resulting in an empty result set.

Leaving the user with the question why his changes were not activated. To be more consistent, the situation is now detected and an error is raised, explaining the situation to the user:

There are changes to activate, but no site can be activated (The sites A, B, C have changes, but may be offline or not logged in).

The caller needs wait and retry the operation at a later time once the sites in question are reachable again.

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