Subclass CV exception from StandardError
Split up form container's validity checks

Eases the addition of new validity checking if we need it
Properly translate RDF controlled vocabulary data

Ensures datastreams are proper RDF triples instead of using simple
strings to represent the URIs
Refactor form-to-attribute conversion
Update metadata-ingest-form gem
Push CV errors to ingest form UI
Merge pull request #199 from OregonDigital/feature/handle-cv-free-entry-exception

Feature/handle cv free entry exception