Stats call doesn't handle I/O error

Repeat as follows:

- Call to MemcachedClient.Stats(type)
- This builds a StatsOperation for each node and asynchronously performs MemcachedNode.Execute(cmd)
- An I/O error occurs, so StatsOperation leaves its member "results" as null and Execute returns false to indicate an error
- MemcachedClient ignores the returned Boolean value from EndInvoke and puts a null dictionary into its own set of results
- ServerStats.GetRaw(key) dies trying to select into the missing dictionary

Plausible fix: check the status from EndInvoke and don't stick anything in the dictionary if there was an error.