In cases where an actor's transport server task (by default handling new
TCP connections) terminates early but does not de-register from the
pertaining registry (aka the registrar) actor's address table, the
trying-to-connect client actor will get a connection error on that
address. In the case where client handles a (local) `OSError` (meaning
the target actor address is likely being contacted over `localhost`)
exception, make a further call to the registrar to delete the stale
entry and `yield None` gracefully indicating to calling code that no
`Portal` can be delivered to the target address.
This issue was originally discovered in `piker` where the `emsd`
(clearing engine) actor would sometimes crash on rapid client
re-connects and then leave a `pikerd` stale entry. With this fix new
clients will attempt connect via an endpoint which will re-spawn the
`emsd` when a `None` portal is delivered (via `maybe_spawn_em()`).
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