Regression test: pbc-leak in anonymous list objects for a non-existent bucket

cf. https://github.com/basho/riak_cs/issues/985
Fix pbc-leak in anonymous list objects for a non-existent bucket
Regression test: pbc-leak in /riak-cs/users request

cf. https://github.com/basho/riak_cs/issues/985
Fix pbc-leak in /riak-cs/users requests

This also fixes premature pbc checkin to the pool.
See also: https://github.com/basho/riak_cs/commit/60b32b3e06a2959d815a602ebe5e23f7eef4c104
Remove dummy errror
Fix return type mismatch
Change multibag repository reference to master branch
Pin the fix on multibag
Merge release/1.5 to avoid conflict
Merge pull request #986 from basho/bugfix/pbc-pool-leak

Fix PB connection pool (pbc_pool_master) leaks

Reviewed-by: kuenishi