Handle errors from rummager/router callbacks
Handle errors from rummager/router callbacks

On first attempt in preview, one of the Artefacts 404d, most likely
from the router. These changes mean that we will continue the import.
The artefact will have been saved anyway.

theodi/panopticon,alphagov/panopticon,theodi/panopticon,theodi/panopticon,theodi/panopticon,alphagov/panopticon,alphagov/panopticon,alphagov/panopticon