Add Errno::EHOSTUNREACH to possible CannotConnectError failures

Hello,

I'm using the redis-sentinel gem to handle possible server failures, but have hit an issue when a server goes down.
When a server experiences a power failure or reboots, the next request will raise Errno::EHOSTUNREACH, which is not expected.
This looks like a logical candidate to be converted into a CannotConnectError.

A way to reproduce this exception is to initialize a Redis client to a closed port or a non-existant IP on your local network and then make any request on it.