Library(redis): report DNS failure

In testing library(redis) I mistyped the redis host name in redis_server/3, and when I ran the redis/2 command it simply blocked forever. Problably should throw an exception like ping does:

$ ping: nonexistent
ping: nonexistent: No address associated with hostname

As is, redis_server/3 does not check the server address. Calls to redis/3 and anything that depend on it are retried after 10ms, doubling on each failure up to one minute. It then keeps trying for a day. You can change these defaults using library(setting).

The question is when to retry and when not. Is a DNS failure sufficiently different from a failed server which may also be permanently failed or inaccessible due to routing or firewall issues.

We could of course let redis_server/3 do a DNS check. On the other hand, a rebooting cluster may not always have its DNS right from the start. A possible intermediate could be to print some or all connect failures as warnings.