Fix return value check for anetTcpAccept

anetTcpAccept returns ANET_ERR, not AE_ERR.

This isn't a physical error since both ANET_ERR
and AE_ERR are -1, but better to be consistent.
Remove some redundant code

Function nodeIp2String in cluster.c is exactly
anetPeerToString with a pre-extracted fd.
Remove redundant IP length definition

REDIS_CLUSTER_IPLEN had the same value as
REDIS_IP_STR_LEN.  They were both #define'd
to the same INET6_ADDRSTRLEN.
Update cluster accept error to error condition

If we woke up to accept a connection, but we can't
accept it, inform the user of the error going on
with their networking.