Improve logging so we can tell which vnode failed to start up.

Was an issue resolving zd://6200.

Output after

```
2013-10-02 13:13:45.244 [error] <0.491.0>@riak_kv_vnode:init:381 Failed to start index 365375409332725729550921208179070754913983135744 backend riak_kv_bitcask_backend error: eacces
2013-10-02 13:13:45.249 [notice] <0.491.0>@riak:stop:43 "backend module failed to start."
```