pgsql: Fix wording in amvalidate error messages

Fix wording in amvalidate error messages

Remove some gratuituous message differences by making the AM name
previously embedded in each message be a %s instead. While at it, get
rid of terminology that's unclear and unnecessary in one message.

Discussion: https://postgr.es/m/20170523001557.bq2hbq7hxyvyw62q@alvherre.pgsql

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1849b72169cdfb60794766c6fc6593bcc7c9a936

Modified Files
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src/backend/access/brin/brin_validate.c | 36 ++++++++++++++---------------
src/backend/access/gin/ginvalidate.c | 32 +++++++++++++-------------
src/backend/access/gist/gistvalidate.c | 32 +++++++++++++-------------
src/backend/access/hash/hashvalidate.c | 40 ++++++++++++++++-----------------
src/backend/access/nbtree/nbtvalidate.c | 36 ++++++++++++++---------------
src/backend/access/spgist/spgvalidate.c | 36 ++++++++++++++---------------
6 files changed, 106 insertions(+), 106 deletions(-)