Don't delete current object version on bad md5

It [looks likes](https://github.com/basho/riak_cs/blob/bc99be0d4c62947a36cc7932f8db8173b54a4d68/src/riak_cs_put_fsm.erl#L290-L303) we delete the currently active manifest on an upload if the md5 is wrong, instead of just never putting this one into the active state.