address unmatched_returns in cluster metadata code

the following dialyzer errors no longer appear with this commit:

```
hashtree_tree.erl:343:
Expression produces a value of type hashtree:hashtree(), but this value is unmatched

hashtree_tree.erl:360:
Expression produces a value of type hashtree:hashtree(), but this value is unmatched

hashtree_tree.erl:368:
Expression produces a value of type hashtree:hashtree(), but this value is unmatched

hashtree_tree.erl:376:
Expression produces a value of type hashtree_tree:tree(), but this value is unmatched

riak_core_broadcast.erl:301:
Expression produces a value of type [['ok' | [any()]]], but this value is unmatched

riak_core_broadcast.erl:326:
Expression produces a value of type 'ok' | ['ok' | ['ok' | [any()]]], but this value is unmatched

riak_core_broadcast.erl:334:
Expression produces a value of type 'ok' | ['ok' | ['ok' | [any()]]], but this value is unmatched

riak_core_broadcast.erl:338:
Expression produces a value of type 'ok' | ['ok' | ['ok' | [any()]]], but this value is unmatched

riak_core_broadcast.erl:351:
Expression produces a value of type 'ok' | ['ok' | ['ok' | [any()]]], but this value is unmatched

riak_core_broadcast.erl:383:
Expression produces a value of type 'ok' | ['ok' | ['ok' | [any()]]], but this value is unmatched

riak_core_broadcast.erl:452:
Expression produces a value of type ['true'], but this value is unmatched

riak_core_metadata_exchange_fsm.erl:184:
Expression produces a value of type ['ok'], but this value is unmatched

riak_core_metadata_manager.erl:332:
Expression produces a value of type atom() | tid(), but this value is unmatched

riak_core_metadata_manager.erl:337:
Expression produces a value of type {'ok','cluster_meta_manifest'}, but this value is unmatched

riak_core_metadata_manager.erl:402:
Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched

riak_core_metadata_manager.erl:560:
Expression produces a value of type atom() | tid(), but this value is unmatched

riak_core_metadata_manager.erl:599:
Expression produces a value of type atom() | {'error',_} | tid(), but this value is unmatched
```

Review:
* [x] verify riak_test `cluster_meta_basic` still passes (NOTE: there is known, unrelated intermittent failure)
* [x] verify dialyzer errors above are gone
* [x] verify there were no missed dialyzer errors in cluster metadata files

A useful grep pattern for review: `"\(riak_core_\(broadcast\|metadata\)\|hashtree_tree\)"`