pgstatindex, pageinspect: handle partitioned indexes

pgstatindex, pageinspect: handle partitioned indexes

Commit 8b08f7d4820f failed to update these modules to at least give
non-broken error messages for partitioned indexes.  Add appropriate
error support to them.

Peter G. was complaining about a problem of unfriendly error messages;
while we haven't fixed that yet, subsequent discussion let to discovery
of these unhandled cases.

Author: Michaël Paquier
Reported-by: Peter Geoghegan
Discussion: https://postgr.es/m/CAH2-WzkOKptQiE51Bh4_xeEHhaBwHkZkGtKizrFMgEkfUuRRQg@mail.gmail.com