Invalid hintfile error message is too severe

Invalid hintfiles happen all the time and it's ok. We should use warning or notice for:

```
2014-04-16 16:45:46.838 [error] <0.2110.0> Hintfile './data/fs_chunks/1370157784997721485815954530671515330927436759040/3.bitcask.hint' invalid
```