Better logging when worker is added when cluster is not ready

[superseb]

What kind of request is this (question/bug/enhancement/feature request):
enhancement

Steps to reproduce (least amount of steps as possible):

1. Create custom cluster and add etcd + controlplane node
2. Make cluster go in Updating/Unavailable state by editing cluster or removing cluster/node agents
3. Add a worker node will cluster is in this state

Agent logging will show failed to find plan for. We can add logging why this is the case, and investigate if we can block registering when the cluster is not available but we have to check recovery situations before we implement this.

[stale]

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