refactor: move checkIsFatal logic to usePool

[steve-chavez]

Towards solving https://github.com/PostgREST/postgrest/issues/3523

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.