Tweak libpq so that if a backend ERROR message arrives while libpq

Tweak libpq so that if a backend ERROR message arrives while libpq
thinks the connection is idle, the error message is displayed as if
it were a NOTICE.  This seems better than dropping the message on
the floor ... particularly if the message is the backend telling us
why it's about to close the connection.  The previous behavior was
Backend message type 0x45 arrived while idle
pqReadData() -- backend closed the channel unexpectedly.
which is not real helpful.