Move it to class 57 (Operator Intervention), which seems like a
better choice given that from the client's standpoint it behaves
a heck of a lot like, e.g., ERRCODE_ADMIN_SHUTDOWN.

In a green field I'd put ERRCODE_IDLE_IN_TRANSACTION_SESSION_TIMEOUT
here as well.  But that's been around for a few years, so it's
probably too late to change its SQLSTATE code.

Discussion: https://postgr.es/m/763A0689-F189-459E-946F-F0EC4458980B@hotmail.com