AS-288: Check deleted CLB when adding node - no tests (review)

This will reduce unwanted retries which will in-turn succeed invalid load balancer tests. 

I have factored out the checking of deleted CLB in separate function. Tests are not changed yet. Kindly review it and I'll change tests after that.
Okies. I was thinking of storing `RequestError` instance in the exception. This way it can be inspected and its `message` can be appended here. What do you think?