When closure of the backend connection is detected during pqFlush,

When closure of the backend connection is detected during pqFlush,
do the right thing: look for a NOTICE message from the backend before we
close our side of the socket.  6.4 libpq did not reliably print the backend's
hara-kiri message, 'The Postmaster has informed me ...', because it only
did the right thing if connection closure was detected during a read
attempt instead of a write attempt.