* Store users: Ensure the last owner cannot be downgraded
Changes the behaviour of the `AddOrUpdateStoreUser` method to throw errors for the failure cases, so that the UI and API can report the actual problem. A role change might fail if the user already has that role or if they are the last owner of the store.
* Cleanup code
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Co-authored-by: nicolas.dorier <nicolas.dorier@gmail.com>
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