The IF EXISTS option was documented, and implemented in the grammar, but
it didn't actually work for lack of support in does_not_exist_skipping().
Per bug #14160.
Report and patch by Kouhei Sutou
Report: <20160527070433.19424.81712@wrigleys.postgresql.org>
Some cases were still reporting errors and aborting, instead of a NOTICE
that the object was being skipped. This makes it more difficult to
cleanly handle pg_dump --clean, so change that to instead skip missing
objects properly.
Per bug #7873 reported by Dave Rolsky; apparently this affects a large
number of users.
Authors: Pavel Stehule and Dean Rasheed. Some tweaks by Álvaro Herrera