Bugfix: Skip invalid state manifests in GC bucket

Addresses #827 partly.

At present, the root cause why active state manifests are entered in GC bucket.
This PR does not process such manifests but just skip and log in order to
GC can proceed to other manifests instead of abort.

It's noteworthy that if `riak_cs_delete_fsm` replies ok with the same total blocks and deleted blocks
to `riak_cs_gc_worker`, then the worker attempt to delete manifest entry in `twop_set`.
Doing it *before* deleting manfiests and blocks completely poses orphan manifests/blocks.
In this PR, `riak_cs_delete_fsm` replies with total blocks = 1 and deleted_blocks = 0.

To address #827 completely, further investigation on root cause needed. Information of logs
by this PR will help.

Without no prior knowledge, this comments are insufficient and sounds like this should stop with  `{stop, {error, invalid_state}, ...}` . We also have to not about #827 , which is like this:

```
  #827 prevents all other blocks and manifests being collected. 
  For now delete FSM stops with pseudo-normal termination to
  let other valid manifests be collected, as the root cause of #827
  is still unidentified.
```