HINT fields to a user-thrown error message, and to specify the SQLSTATE
error code to use.  The syntax has also been tweaked so that the
Oracle-compatible case "RAISE exception_name" works (though you won't get a
very nice error message if you just write that much).  Lastly, support
the Oracle-compatible syntax "RAISE" with no parameters to re-throw
the current error from within an EXCEPTION block.

In passing, allow the syntax SQLSTATE 'nnnnn' within EXCEPTION lists,
so that there is a way to trap errors with custom SQLSTATE codes.

Pavel Stehule and Tom Lane