Add HEAD requests against report urls for --dry_run

This is what I've been using locally for the past few scrapers.

I don't think `check_report_url` is a particularly great name.

It's also possible that we should allow other status codes (301, 302, etc). Maybe use `res.ok` instead?

Closes #108
You are correct. I thought we still required a `url` value of `None` for unreleased reports, but that does not seem to be the case.