prettifying ValidationErrors makes them less useful

I often get this sort of traceback in tests:

```
ValidationError: {u'Resources': u'Package resource(s) invalid', u'  type': u'V'}
```

There is no field named "Resources" and what's "type" thing with two spaces in front? With this change I now get:

```
ValidationError: {u'__type': u'Validation Error', u'resources': [{u'resource_type': [u'Missing value'], u'language': [u'Missing value']}]}
```

Look at that!  I can tell what fields failed validation and everything.  Beautiful!