New kwarg for autopopulate - returning success count after the `.populate()` call

[ttngu207]

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Introduce a new argument return_success_count to the .populate() routine, that would return the count of successful make() calls in one .populate() call.

This functionality allows for us to answer the question: "Did anything happen in my .populate() call?" without having to call the key_source again (e.g. with .progress())