Normalize the return value from the loop.
Ensure statistics can be registered.

Resolves:

riak_kv_stat.erl:256: Expression produces a value of type 'ok' | {'error',_,'unsupported_metric_type'}, but this value is unmatched
riak_kv_stat.erl:288: Expression produces a value of type 'ok' | {'error',_,'unsupported_metric_type'}, but this value is unmatched