final::validate() hook

[wbond]

Ticket #601 by @jeffturcotte:

Would be nice to have a validation hook that gets called AFTER message reordering and replacement. Alternatively, post::validate could be moved, or message reordering and replacement could simply use post::validate() instead of being standalone. But both of those require more work and potential compatibility breaks.

(for reference fActiveRecord, lines 2901-2903)