Local Stats cache stores error message and doesn't regenerate after a timeout or error condition [JIRA: TOOLS-81]

[mk1s]

In certain instances when Riaknostic encounters a timeout or error that satisfies the {ok, Stats} pattern match, this message is stored in the riaknostic -> local_stats environment variable. This message is returned as the current Stats output which causes Riaknostic to crash. The current workaround is to issue a application:unset_env(riaknostic, local_stats) in order to clear the cache and have the stats regenerated.

I suggest reworking https://github.com/basho/riaknostic/blob/develop/src/riaknostic_node.erl#L194 to include more specific matching so the stats cache can be cleared or regenerated under these error conditions.