fix: remove red herring warp logs on default log-level

The logs added on e95e815483 are red
herrings under normal operation.

This moves them to `log-level=debug` and removes "error" from the
message prefix.

Fixes https://github.com/PostgREST/postgrest/issues/4799

src/PostgREST/Observation.hs

@@ -57,7 +57,7 @@ data Observation
   | JwtCacheLookup Bool
   | JwtCacheEviction
   | TerminationUnixSignalObs Text
-  | WarpErrorObs Text
+  | WarpServerObs Text
   deriving (Generic)
 
 data ObsFatalError = ServerAuthError | ServerPgrstBug | ServerError42P05 | ServerError08P01