fix: catch 404 errors in AA regions requests

[bengesoff]

The subscriptions requests return a custom error type when the API returns a 404. The regions requests can also receive 404s when the subscription doesn't exist, so should also return the custom error type. Given that this corresponds to subscription not found errors, I have reused the same custom type in the subscriptions package.

This change results in a minor change to the existing error type, but it is not a breaking change as it changes a property from private to public, so nothing could be depending on it already