Error when entity reference is stale

I actually think this is good enough, the long IF is not nice, but works fine.

@fago I tested and it works well. I left a comment in an old thread that I think is still actual. Could you confirm?
Because that comment is a little unrelated to this bug report issue I merge this PR and depending on your answer I can quickly add a follow-up commit or even a follow-up issue.

> if the referenced entity is not accessible now but later will become accessible then the parent custom element must know about the change?

agreed, yes. I was thinking it's using our entityIsAccessible() helper now, but apparently still doesn't. Let's fix that!

@fago Created a small PR, locally tested and seems to work well. I think we can merge and finish here?

agreed, looks good.

Merged, thanks everybody!

Automatically closed - issue fixed for 2 weeks with no activity.