firewaller and remote-relations workers fail if external controllers are unavailable

If you relate two applications in different controllers (i.e. cross-model, cross-controller), and stop (or kill) the offering controller, then the firewaller and remote-relations workers in the consuming controller will go into an error loop.

machine-0: 17:43:19 ERROR juju.worker.dependency "remote-relations" manifold worker returned unexpected error: handling change for relation "wordpress:db mysql:db": opening facade to remote model: try was stopped
machine-0: 17:43:22 ERROR juju.worker.dependency "firewaller" manifold worker returned unexpected error: cannot open facade to remote model to publish network change: try was stopped

Neither should do this. The firewaller will not be able to manage intra-model firewall changes. The remote-relations worker will not be able to process changes for remote relations for other, accessible, controllers.