Add an artefact! method that can raise exceptions

After 68c5e678 was merged, .artefact will just return nil when it has
been archived. Certain Apps (like Frontend) expect a 410 to be returned,
so this commit adds artefact! which will raise an exception.
Merge pull request #141 from alphagov/add-artefact-bang

Add an artefact! method that can raise exceptions