broken/departed are not handled properly

While reviewing this Merge Proposal:

https://code.launchpad.net/~cjwatson/charms/precise/postgresql/broken-departed-hooks/+merge/135132

I noticed that the hooks.py script in the charm is badly in need of a refactor to handle departed/broken.

While the change above is suppressing the error, its definitely just a band-aid.

Departed happens when only one of the units of the related service is removed. broken happens when the entire relationship is removed.

In a departed hook, you have access to the unit that departed, and can remove just the resources that it was consuming (such as a GRANT or IP-specific access controls). With broken, you are obligated to tear all of these down, and can only do so using $JUJU_RELATION_ID, as relation-list will not function.

So when I read hooks.py, I see db-relation-broken being, no pun intended, badly broken, as it is doing a relation-get, which is really not possible at all. It does a bare try: except:, which is really bad form in any python program, but in this case is masking the fact that broken is most definitely failing every time.

A refactor around $JUJU_RELATION_ID is needed to make this charm reliable.. and it appears that basically on every relation hook, the whole config should just be regenerated using relation-ids/relation-list/relation-get.