pgsql: Escalate ERRORs during async notify processing to FATAL

Escalate ERRORs during async notify processing to FATAL

Previously, if async notify processing encountered an error, we would
report the error to the client and advance our read position past the
offending entry to prevent trying to process it over and over
again. Trying to continue after an error has a few problems however:

- We have no way of telling the client that a notification was
  lost. They get an ERROR, but that doesn't tell you much. As such,
  it's not clear if keeping the connection alive after losing a
  notification is a good thing. Depending on the application logic,
  missing a notification could cause the application to get stuck
  waiting, for example.

- If the connection is idle, PqCommReadingMsg is set and any ERROR is
  turned into FATAL anyway.

- We bailed out of the notification processing loop on first error
  without processing any subsequent notifications. The subsequent
  notifications would not be processed until another notify interrupt
  arrives. For example, if there were two notifications pending, and
  processing the first one caused an ERROR, the second notification
  would not be processed until someone sent a new NOTIFY.

This commit changes the behavior so that any ERROR while processing
async notifications is turned into FATAL, causing the client
connection to be terminated. That makes the behavior more consistent
as that's what happened in idle state already, and terminating the
connection is a clear signal to the application that it might've
missed some notifications.

The reason to do this now is that the next commits will change the
notification processing code in a way that would make it harder to
skip over just the offending notification entry on error.

Reviewed-by: Matheus Alcantara <matheusssilv97@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Arseniy Mukhin <arseniy.mukhin.dev@gmail.com>
Discussion: https://www.postgresql.org/message-id/fedbd908-4571-4bbe-b48e-63bfdcc38f64@iki.fi
Backpatch-through: 14

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/aab4a84bb070ae73dcd72d25c2239de50c6150ab

Modified Files
--------------
src/backend/commands/async.c | 34 +++++++++++++++++++++-------------
1 file changed, 21 insertions(+), 13 deletions(-)