schedule manifests and watch the intent store

@mpuncel @petertseng These are useful if you want to schedule something in the intent store and observe that it made it. `p2-watch` will continue to observe the intent store for changes, just as the preparer does.
Well maybe I should have looked closer, but what I should have said is:
if `err != nil`, then necessarily `manifest == nil`.
And so by falling through we are going to dereference nil.