Code review comment for lp:~rvb/maas/retry-power-changes

Good branch but a few things to fix:

1. Use the test matchers, Luke! (See inline)
2. You're not catching Failures from the deferToThread calls (yield turns errback into exceptions)
3. The tests aren't very Twisted-like in terms of using a Clock(), which means that you've not tested the pause durations anywhere. If you don't know how to use Clock() ping me on IRC and I'll show you.