Return a 'forbidden' error when BulkDocs fails to save an entity.

When _bulk_docs save happens, sometimes an entity will fail to save with an exception.

When this happens the response should return an error of type "Forbidden" to comply with the couchdb spec.
https://docs.couchdb.org/en/stable/api/database/bulk-api.html#bulk-docum...

Returning any other codes can mean that the response isn't counted properly as a failure by certain clients.