clusterLoadConfig() REDIS_ERR retval semantics refined.
clusterLoadConfig() REDIS_ERR retval semantics refined.

We should return REDIS_ERR to signal we can't read the configuration
because there is no config file only after checking errno, othewise
we risk to rewrite an existing file that was not accessible for some
other reason.

JackieXie168/redis,JackieXie168/redis,JackieXie168/redis,JackieXie168/redis