Errors on schema validation should print full path to invalid field

If I receive the following validation error:

```
class mismatch for id: expected [<type 'str'>, <type 'unicode'>], 1 has class int
```

I'm not sure _where_ in my document its referring to the `id` field. It'd be great if the output indicated, say, the full JSON path of the field (e.g., `certificate.parsed.certificate_policies.id`)

I could be using the tool incorrectly, so bear with me 😄 