Explicitly error if the category is not found

[G-Rath]

Currently we always assume that the results include the category that was requested, but technically that's not the case - i.e. Lighthouse v12 removed the PWA category which we were using in an app, meaning it started erroring very generically due to trying to do nil * 100.

Lighthouse does not seem to validate the values passed which I've opened an issue for improving, but ultimately we should handle this gracefully too.

I've got a basic patch for this already:

  def category
    category = results.dig('categories', @audit.to_s)

    raise Error, "Category '#{@audit}' not found in Lighthouse results - maybe it was removed?" if category.nil?

    category
  end