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