refactor: move checkIsFatal logic to usePool

The fatal logic is now inside `usePool`. It centralizes the
logic which is better for Locality of Behavior.

Removes:

- The need to do checkIsFatal on other parts of the code
- SCFatalFail/ConnFatalFail states which are no longer needed.

src/PostgREST/Metrics.hs

@@ -47,7 +47,7 @@ observationMetrics (MetricsState poolTimeouts poolAvailable poolWaiting _ schema
   SchemaCacheLoadedObs resTime -> do
     withLabel schemaCacheLoads "SUCCESS" incCounter
     setGauge schemaCacheQueryTime resTime
-  SchemaCacheNormalErrorObs _ -> do
+  SchemaCacheErrorObs _ -> do
     withLabel schemaCacheLoads "FAIL" incCounter
   _ ->
     pure ()