pgsql: Improve a number of elog messages for not-supposed-to-happen

Log Message:
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Improve a number of elog messages for not-supposed-to-happen cases in btrees,
since these seem to happen after all in corrupted indexes.  Make sure we
supply the index name in all cases, and provide relevant block numbers where
available.  Also consistently identify the index name as such.

Back-patch to 8.2, in hopes that this might help Mason Hale figure out his
problem.

Modified Files:
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    pgsql/src/backend/access/nbtree:
        nbtinsert.c (r1.162 -> r1.163)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtinsert.c?r1=1.162&r2=1.163)
        nbtpage.c (r1.104 -> r1.105)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtpage.c?r1=1.104&r2=1.105)
        nbtsearch.c (r1.114 -> r1.115)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtsearch.c?r1=1.114&r2=1.115)