When I began removing `convenience` from actor inits, I thought it would
be ok for an actor's extensions to be non-delegating. But that's wrong,
because the actor could be in a resilient module and still have its
properties change between stored and computed, meaning that there isn't
a practical way to allow this, unless if the extension and the actor
are in the same file.
For now, just re-ban this behavior before anybody notices :)