pgsql: Improve ERROR/LOG messages added by commits ddd5f4f54a and 7a424

Improve ERROR/LOG messages added by commits ddd5f4f54a and 7a424ece48.

Additionally, in slotsync.c, replace one StringInfoData variable usage
with a constant string to avoid palloc/pfree. Also, replace the inclusion
of "logical.h" with "slot.h" to prevent the exposure of unnecessary
implementation details.

Reported-by: Kyotaro Horiguchi, Masahiko Sawada
Author: Shveta Malik based on suggestions by Robert Haas and Amit Kapila
Reviewed-by: Kyotaro Horiguchi, Amit Kapila
Discussion: https://postgr.es/m/20240214.162652.773291409747353211.horikyota.ntt@gmail.com
Discussion: https://postgr.es/m/20240219.134015.1888940527023074780.horikyota.ntt@gmail.com
Discussion: https://postgr.es/m/CAD21AoCYXhDYOQDAS-rhGasC2T+tYbV=8Y18o94sB=5AxcW+yA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/801792e528d6cb5eeb3a70f5ceb10b15bf92f0c7

Modified Files
--------------
src/backend/replication/logical/slotsync.c         | 65 +++++++++-------------
.../recovery/t/040_standby_failover_slots_sync.pl  |  2 +-
2 files changed, 28 insertions(+), 39 deletions(-)