pretty print errors

Right now, JSCK produces errors like this:

```
[ { schema:
     { pointer: '#/definitions/cors/anyOf',
       attribute: 'anyOf',
       definition: [Object] },
    document: { pointer: '#/aws/cors', path: 'aws.cors', value: 'default' } } ]
```
We should be able to add a pretty printing capability for these errors.