pgsql: Tidyup WARNING ereports in subscriptioncmds.c

Tidyup WARNING ereports in subscriptioncmds.c

A couple of ereports were making use of StringInfos as temporary storage
for the portions of the WARNING message. One was doing this to avoid
having 2 separate ereports. This was all fairly unnecessary and
resulted in more code rather than less code.

Refactor out the additional StringInfos and make
check_publications_origin_tables() use 2 ereports.

In passing, adjust pubnames to become a stack-allocated StringInfoData to
avoid having to palloc the temporary StringInfoData. This follows on
from the efforts made in 6d0eba662.

Author: Mats Kindahl <mats(dot)kindahl(at)gmail(dot)com>
Reviewed-by: David Rowley <dgrowleyml(at)gmail(dot)com>
Reviewed-by: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Reviewed-by: Álvaro Herrera <alvherre(at)kurilemu(dot)de>
Discussion: https://postgr.es/m/0b381b02-cab9-41f9-a900-ad6c8d26c1fc%40gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/448b6a4173d007c75ba30fed666b60f0bd1afe8b

Modified Files
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src/backend/commands/subscriptioncmds.c | 59 +++++++++++++++------------------
1 file changed, 26 insertions(+), 33 deletions(-)