Geo: Refactor: Raise if selective sync type unknown

What does this MR do and why?

Small refactor prior to !201213 (merged).

Refactor since an unknown selective sync type is truly an exceptional case-- there is no way to reach that case in the application.

I do this in order to make it easy for the FF to disable new behavior even if the selective sync type is left as "organizations" in: !201213 (merged)

References

Related to #514251 (closed)

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