pageinspect

The contrib extensions pageinspect, pg_visibility and pgstattuple only
work against regular relations which have storage.  They don't work
against foreign tables, partitioned (parent) tables, views, et al.

Add checks to the user-callable functions to return a useful error
message to the user if they mistakenly pass an invalid relation to a
function which doesn't accept that kind of relation.

In passing, improve some of the existing checks to use ereport() instead
of elog(), add a function to consolidate common checks where
appropriate, and add some regression tests.

Author: Amit Langote, with various changes by me
Reviewed by: Michael Paquier and Corey Huinker
Discussion: https://postgr.es/m/ab91fd9d-4751-ee77-c87b-4dd704c1e59c@lab.ntt.co.jp