pgsql: Fix an oversight in 3f28b2fcac.

Fix an oversight in 3f28b2fcac.

Commit 3f28b2fcac tried to ensure that the replication origin shouldn't be
advanced in case of an ERROR in the apply worker, so that it can request
the same data again after restart. However, it is possible that an ERROR
was caught and handled by a (say PL/pgSQL) function, and the apply worker
continues to apply further changes, in which case, we shouldn't reset the
replication origin.

Ensure to reset the origin only when the apply worker exits after an
ERROR.

Commit 3f28b2fcac added new function geterrlevel, which we removed in HEAD
as part of this commit, but kept it in backbranches to avoid breaking any
applications. A separate case can be made to have such a function even for
HEAD.

Reported-by: Shawn McCoy <shawn(dot)the(dot)mccoy(at)gmail(dot)com>
Author: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Reviewed-by: Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
Reviewed-by: vignesh C <vignesh21(at)gmail(dot)com>
Reviewed-by: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Backpatch-through: 16, where it was introduced
Discussion: https://postgr.es/m/CALsgZNCGARa2mcYNVTSj9uoPcJo-tPuWUGECReKpNgTpo31_Pw@mail.gmail.com

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/0de091a4b585bfb3ffa2dc1f6e7465db1d0b3eba

Modified Files
--------------
src/backend/replication/logical/worker.c | 21 +++++----
src/test/subscription/t/100_bugs.pl | 77 ++++++++++++++++++++++++++++++++
2 files changed, 87 insertions(+), 11 deletions(-)