meter-status worker spins if charm doesn't have meter-status-changed hook

[John A Meinel]

I was just trying out a prototype charm with juju-1.26 master (48f09ba4).

What I end up seeing is that every 3 seconds I get an:
  ERROR juju.worker.dependency engine.go:476 "meter-status" manifold worker returned unexpected error: error running 'meter-status-changed' hook: /var/lib/juju/agents/*/charm/hooks/meter-status-changed does not exist.

While it is true that 'meter-status' cannot actually run a meter-status-changed hook, at best it should be a WARNING (more likely INFO/DEBUG) and it should go back to being quiet.

Filling up the logs every 3s because it can't run an unsupported hook is bad mojo. If this was a production environment, we'd log ourselves to death rather quickly on all currently existing charms.