Add diagnostic sensor for unknown entities

[klaptafel]

Checklist

• I checked for similar existing requests (both open and closed) before posting.
• My request is generic, other users may benefit from it too.

Proposal

When a entity changes or becomes unknown, alarmo stops working. I would love to see a diagnostic sensor for alarmo which senses unknown entities. That way you could use this in automations and f.e. notify when this is a problem.

Additional info

I guess a binary_sensor which shows a problem is a good start.