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/App.hs

@@ -99,7 +99,7 @@ run appState = do
     onWarpException :: Maybe Wai.Request -> SomeException -> IO ()
     onWarpException _ ex =
       when (shouldDisplayException ex) $
-        observer $ WarpErrorObs $ show ex
+        observer $ WarpServerObs $ show ex
 
     -- Similar to wai defaultShouldDisplayException in
     -- https://github.com/yesodweb/wai//blob/8c3882c60f6abe043889fc20c7efd3fa9747fa4a/warp/Network/Wai/Handler/Warp/Settings.hs#L251-L258