pgsql: Improve error message for pg_create_subscription.

Improve error message for pg_create_subscription.

c3afe8cf5a updated this error message, but it didn't use the new
style established in de4d456b40.

Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/20230512203721.GA2644063%40nathanxps13.home

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4d5105a684ba20bf6a98e6ae48224fe93382c040

Modified Files
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src/backend/commands/subscriptioncmds.c    | 4 +++-
src/test/regress/expected/subscription.out | 3 ++-
2 files changed, 5 insertions(+), 2 deletions(-)