Errors shouldn't block receiving fact check

www.agileplannerapp.com/boards/173808/cards/5005

the edition mentioned in the story had invalid links in one of its parts. errors were causing it to remain in 'fact_check' even after calling event 'receive_fact_check'.

it seems unnecessary to validate the document when we're receiving fact check response, hence this change.
i had inferred it from the message the gem owner posted on [this discussion](http://rubydoc.info/github/pluginaweek/state_machine/StateMachine/Machine:event). however, i've replaced it [https://github.com/alphagov/govuk_content_models/commit/09b56a0526343fd14badf66b473769dd641e155a] to avoid using an undocumented feature. thanks for pointing out!