Log errors in fact check processing

We already email them to govuk-dev, so this will just record them locally.

Looking at the production logs, it appears to be sending those emails, but they
are not visible in the Google Group.

I suspect that the Google Group might have marked them as spam so they are not
showing upon browsing/searching the govuk-dev group, and I want to find out if
something is going wrong.
Merge pull request #87 from alphagov/log_fact_check_errors

Log errors in fact check processing