More visibile storage errors

I've been monkeypatching this for a few months to integrate with bugsnag. This way, I can:

```javascript
    var lastStorageError = Batman.RailsStorage.get('lastError')
    var lastStorageErrorIsAbort = Batman.RailsStorage.get('lastError.request.status') == 0 
    if (lastStorageErrorIsAbort) {
      return false
    }
    else if (lastStorageError) {
      metaData.lastStorageError = lastStorageError.toJSON()
    }
```

And I get all the request info in bugsnag. Crucial!