Fix extended statistics with partial analyzes

Fix extended statistics with partial analyzes

Either because of a previous ALTER TABLE .. SET STATISTICS 0 or because
of being invoked with a partial column list, ANALYZE could fail to
acquire sufficient data to build extended statistics.  Previously, this
would draw an ERROR and fail to collect any statistics at all (extended
and regular).  Change things so that we raise a WARNING instead, and
remove a hint that was wrong in half the cases.

Reported by: David Rowley
Discussion: https://postgr.es/m/CAKJS1f9Kk0NF6Fg7TA=JUXsjpS9kX6NVu27pb5QDCpOYAvb-Og@mail.gmail.com