Propagate an error tuple and add error logs

- Don't throw away an error tuple, propagate it and match
- Add error log via lager because output to terminal is often lost
Add test case to cleanup orphan MPs in 30 buckets

sum(pool sizes) < 30
Fix riak_client leak, stop it after use
Merge pull request #933 from basho/bugfix/pbc-leak

Fix pbc leak in cleaning orphan MPs

Reviewed-by: kuenishi