take SQL INSERT dependence on "post-fetching rows" out of SQL, make it ORM specific

[Mike Bayer]

right now the engine insists that it be able to get back a row that was inserted. since postgres/psycopg makes this so difficult, take this requirement out, make it so that only the ORM raises an error if it cant get a row back from the engine.

[Mike Bayer]

mostly a postgres issue, fixed in changeset:997. the "cant get the row/no OIDs" exception is raised when you call last_inserted_ids() and there are none. the post_exec just sets it to None if it cant post-load the row.