libpq: Notice errors a backend may have sent just before dying.

At least since the introduction of Hot Standby, the backend has
sometimes sent fatal errors even when no client query was in
progress, assuming that the client would receive it.  However,
pqHandleSendFailure was not in sync with this assumption, and
only tries to catch notices and notifies.  Add a parseInput call
to the loop there to fix.

Andres Freund suggested the fix.  Comments are by me.
Reviewed by Michael Paquier.