Make error logging in extended statistics more consistent

Make error logging in extended statistics more consistent

Most errors reported in extended statistics are internal issues, and so
should use elog(). The MCV list code was already following this rule, but
the functional dependencies and ndistinct coefficients were using a mix
of elog() and ereport(). Fix this by changing most places to elog(), with
the exception of input functions.

This is a mostly cosmetic change, it makes the life a little bit easier
for translators, as elog() messages are not translated. So backpatch to
PostgreSQL 10, where extended statistics were introduced.

Author: Tomas Vondra
Backpatch-through: 10 where extended statistics were added
Discussion: https://postgr.es/m/20190503154404.GA7478@alvherre.pgsql