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