Person Editor: display error if device no longer exists

[SeanPM5]

Home Assistant release with the issue:
0.99.2

Description of problem:
Upgraded my phone the other day and realized today that I forgot to update my "Person" to point to the new device_tracker entity. I noticed that the device_tracker entity from my old phone was still here, and displayed exactly like all of my other devices - despite this entity no longer even existing.

Missing devices should should be visibly communicated to the user in some way. Lovelace views already do this with the yellow "Entity not available" messages and such. I don't think that UI would fit here though (since you need to be able to delete), so I came up with this very quick mockup:

Idea is basically to make the label and UI elements red. Although it's not in this mockup I think the entity ID text should be red also. Purpose is to make the user aware of a missing one so they can either swap it out or delete it. I think that this could be used elsewhere too like the Lovelace card editors.