pgsql: Minor error message enhancement

Minor error message enhancement

In refuseDupeIndexAttach(), change from

    errdetail("Another index is already attached for partition \"%s\"."...)

to

    errdetail("Another index \"%s\" is already attached for partition \"%s\"."...)

so we can easily understand which index is already attached for
partition \"%s\".

Author: Jian He <jian.universality@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion:
https://www.postgresql.org/message-id/flat/CACJufxGBfykJ_1ztk9T%2BL_gLmkOSOF%2BmL9Mn4ZPydz-rh%3DLccQ%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/e9c043a11ac402d376def531a12883d1dac15315

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