Improve error message when "categories" is not specified in metadata

File: [python/model/base.py](https://github.com/lopezco/ml-flask-api/blob/master/python/model/base.py)

Add something like:

```python
elif var_type == 'category':
    if categories is not None:
        var_type = CategoricalDtype(categories=categories, ordered=True)
    else:
        msg = 'Missing "categories" for "{}" in metadata'.format(name)
        raise ValueError(msg)
```