cli: error out gracefully when requiring #6
print src from cli

```
% transform --src iso-date-codec
{ '@context': 'http://transformer.io/context/transformer.jsonld',
  '@type': 'Codec',
  '@id': 'transformer/iso-date-codec',
  description: 'ISO 8601 date format: 2006-01-02T15:04:05Z07:00' }
```