A promise was rejected with a non-error

It seems that cordova-file-cache is rejecting promises with non-error objects. This causes bluebird to print warnings, and it seems to totally mess up the stack trace, which is really really unhelpful.

For example, in one instance it rejects with a list of failed files or something:

```
reject(self.getDownloadQueue());
```

Please can this be fixed?

Thanks.
