Cluster manager should check if operator is running

[jacobtomlinson]

The new experimental cluster manager relies on the operator and CRDs to create the Dask cluster components for us.

We need to ensure that the user experience is good for folks using the cluster manager but do not have the CRDs or operator running.

If no CRDs are installed it should fail early and give a useful error message.

If the operator is not running we need some way to detect that and also give an error message. Perhaps the operator should add a label to the DaskCluster resource to show it has been picked up. We could watch for that label and if it doesn't appear after a timeout we could show a useful error.

[jacobtomlinson]

We should also gracefully handle users not having the right permissions to create resources.