Code review comment for lp:~cjwatson/storm/refactor-exception-wrapping

I've done a full successful Launchpad test run with this patch. The only change I had to make to Launchpad was to adjust LaunchpadDatabase.__init__, which is just because it's subclassing Postgres while (for unrelated reasons) not running the superclass constructor, so as a consequence it has to keep up with changes to the superclass constructor, which I think is fine.