Formally define the different Poller states

Poller now tracks its own state and logs warnings or errors when the
state changes in a way that could cause issues. We've opted for the
following:

  * `initial`: when no data has been fetched and we serve default data
  * `erroring`: when no data has ever been fetched
  * `stale`: when data has been fetched before but is now erroring
  * `fresh`: when fresh data has been fetched with no errors

We included n-logger and now log a warning or error for the `erroring`
or `stale` events.