Vocabularies containing a concept without a skos:prefLabel errors

Ontologies that have a concept but lack a `skos:prefLabel` will crash vocpub on a `KeyError`

To reproduce:

1. Save as crash.ttl
```
PREFIX : <https://example.com/voc/>
PREFIX skos: <http://www.w3.org/2004/02/skos/core#>

:cs a skos:ConceptScheme ;
    skos:prefLabel "My Vocabulary"@en .

:nolabel a skos:Concept ;
    skos:inScheme :cs .
```
2. Run `pylode crash.ttl -p vocpub -o crash.html`
