BUG #15598: PostgreSQL Error Code is not reported when connection terminated due to idle-in-transaction timeout

The following bug has been logged on the website:

Bug reference: 15598
Logged by: Marcin Barczyński
Email address: mba(dot)ogolny(at)gmail(dot)com
PostgreSQL version: 9.6.8
Operating system: Windows 10
Description:

Here is the output on Ubuntu:
```
postgres=# SELECT version();
version

-------------------------------------------------------------------------------------------------------------------------------------
PostgreSQL 9.6.9 on x86_64-pc-linux-gnu (Ubuntu 9.6.9-2.pgdg14.04+1),
compiled by gcc (Ubuntu 4.8.4-2ubuntu1~14.04.4) 4.8.4, 64-bit
(1 row)

postgres=# SET idle_in_transaction_session_timeout=500;
SET
postgres=# BEGIN;
BEGIN
postgres=# SELECT * FROM pg_class;
FATAL: 25P03: terminating connection due to idle-in-transaction timeout
LOCATION: ProcessInterrupts, postgres.c:2996
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Succeeded.
```

PostgreSQL Error Code 25P03 is reported as expected.

I downloaded PostgreSQL 9.6.11 for Windows from
https://www.openscg.com/bigsql/postgresql/installers.jsp/ (referenced at
https://www.postgresql.org/download/windows/),
and the error code is not reported:
```
postgres=# SELECT version();
version
----------------------------------------------------------------------------------------------------------
PostgreSQL 9.6.11 on x86_64-pc-mingw64, compiled by gcc.exe (Rev5, Built by
MSYS2 project) 4.9.2, 64-bit
(1 row)

postgres=# SET idle_in_transaction_session_timeout=500;
SET
postgres=# BEGIN;
BEGIN
postgres=# SELECT * FROM pg_class;
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Succeeded.
```