work on more detailed messages for errors

Working on more detailed error messages, as mentioned in #83
Here is what they look like at the moment:
```
ValidationError: [{}, 3, 'foo', [3]] is not of type 'string'
    Failed validating 'type' in schema:
        {'items': {'allOf': [{'maxLength': 2, 'type': 'string'},
                             {'minimum': 5, 'type': 'integer'},
                             {'items': [{'type': 'string'},
                                        {'type': 'integer'}],
                              'type': 'array'}]},
         'type': ['string']}
    On instance:
        [{}, 3, 'foo', [3]]

ValidationError: {} is not of type 'string'
    Failed validating 'type' in schema['items']['allOf'][0]:
        {'maxLength': 2, 'type': 'string'}
    On instance[0]:
        {}
```