require an error message on table_proc failure replies

require an error message on table_proc failure replies

The error message is not really used, it just gets translated to a
TEMPFAIL, but it allows to have mandatory logging of errors instead
of relying on tables to hopefully log something.

To ease the transition, don't make it mandatory in smtpd(8) yet,
but document it as such.

ok millert@, gilles@