resolver errors say what went wrong, not "no such name"

/v2/resolve answers 200, 400 or 502, and an unregistered name is
  NRAvailable, so no status means "not registered". Mapping 400/404/410 to
  NOT_FOUND made a misconfigured relay deny every name, and hid a relay
  upgraded ahead of its resolver. All three now surface as RESOLVER.

  rslvNotFound would have gone dead, so it counts what its name says: an
  availability answer the encoder downgrades for a session below v22. The
  wire is unchanged.

  A hashed query the registrar cannot name is refused with 502 rather than
  answered with a record named "unknown", which the client rejects anyway.
  NRRUnknown is capped to 32 printable characters again, as the spec says.
  A registered name that is also reserved no longer offers a date it will
  never free up on. rentPrices is registrationPrices throughout, and
  yearPriceUSD is USDCents rather than a bare Int64.

scripts/resolver/README.md

@@ -182,6 +182,11 @@ every query.
 **Set `SNRC_CONTROLLER_<TLD>` wherever `SNRC_REGISTRAR_<TLD>` is.** Without a
 controller there is no oracle, so no name can be priced.
 
+**Upgrade this service before the routers that query it.** Routers from SMP v22
+call `/v2/resolve`, which an older resolver does not serve. Every name then
+answers `ERR NAME RESOLVER "HTTP 404"` until this service is upgraded, while
+`/health` still reports it as ready.
+
 ### Why a name is reserved
 
 A held-back name carries `reasonCode`, the controller's reason, and `reason`, an