Error message for FK referencing deferrable unique/PK

While looking at this report

http://archives.postgresql.org/pgsql-bugs/2009-08/msg00083.php

I spotted another error message which could use improvement:

CREATE TABLE foo(a int PRIMARY KEY DEFERRABLE);
CREATE TABLE bar(a int REFERENCES foo(a));

ERROR: there is no unique constraint matching given keys for
referenced table "foo"

Whereas if you define the FK slightly differently so that it goes
through transformFkeyGetPrimaryKey() instead, you get a much better
error message:

CREATE TABLE bar(a int REFERENCES foo);

ERROR: cannot use a deferrable primary key for referenced table "foo"

The attached patch to transformFkeyCheckAttrs() makes the former case
generate a similar error to the latter.

- Dean