pgsql: Mention the index name in 'could not create unique index' errors,

Log Message:
-----------
Mention the index name in 'could not create unique index' errors,
per suggestion from Rene Gollent.

Modified Files:
--------------
    pgsql/src/backend/utils/sort:
        tuplesort.c (r1.78 -> r1.79)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/sort/tuplesort.c?r1=1.78&r2=1.79)
    pgsql/src/test/regress/expected:
        alter_table.out (r1.103 -> r1.104)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/alter_table.out?r1=1.103&r2=1.104)
        create_index.out (r1.26 -> r1.27)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/create_index.out?r1=1.26&r2=1.27)