pgsql: Improve the error message introduced in commit 87ce27de696.

Improve the error message introduced in commit 87ce27de696.

The error detail message "Replica identity consists of an unpublished
generated column." implies that the entire replica identity is made up of
an unpublished generated column which may not be the case.

Reported-by: Peter Smith
Author: Shlok Kyal
Reviewed-by: Peter Smith, Amit Kapila
Discussion: https://postgr.es/m/CAHut+PuwMhKx0PhOA4APhJTLoBGNykbeCQpr_CuwGT-SkswG5w@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2d0152d6147bb7d12eb120b7e749a323bf91aa6e

Modified Files
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src/backend/executor/execReplication.c    | 4 ++--
src/test/regress/expected/publication.out | 4 ++--
2 files changed, 4 insertions(+), 4 deletions(-)