Fix wording in amvalidate error messages

Alvaro Herrera <alvherre@alvh.no-ip.org>

Commit: e6785a5ca16bfe67b9c74d168ae6e88c6e55c8ac
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2017-05-30T19:45:42Z
Releases: 10.0
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

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