If for some reason there is not conversion for a field raise an exception.

Otherwise the caller receives a None and some weird error can appear
at random location.

I tried to implement a custom field in peewee but when I use it in the admin interface a strange exception is raised:

    TypeError: 'NoneType' object is not iterable

at the following location

```python
 field_name, form_field = self.model_converter.convert(field.model_class, field, None)
```
since the method return a ``None`` value that can't be unpacked in the tuple on the left side.

I think it's more robust to indicate clearly that some unexpected is happened. What do you think?