Fix/round robin zero division

Hi,
I stumbled upon a `ZeroDivisionError` raised by the `connection_pool.RoundRobinSelector`.
I am not sure exactly how it happened but i was testing my apps fall-backs so elasticsearch was not running at the time.
This pull request only fix the `ZeroDivisionError`, but obviously whatever class that use the connection doesn't like a None value and throws an AttributeError, but I am not familiar enough with the code to know if the `ConnectionError` should be raised in get_connection or directly in `RoundRobinSelector`, or what message to use.