Subscription portal update_namespace_name update

What does this MR do and why?

Follow up for !69896 (merged)

Adds logic to Gitlab::SubscriptionPortal::Client.update_namespace_name so it can handle top level errors:

=> {:success=>true, :data=>{"errors"=>[...], ...} }

Resulting from query execution (eg. authentication errors) and respond with failure instead of raising an exception.

🏁 All these changes are behind a feature flag ref: #341056 (closed) so this ~bug is a fix for this feature that is not yet released.

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