Return an error to the callback on 404 responses

Without this, tilelive.copytask will try accessing data.solid with data being null, and then crash out.

The copytask is happy to skip missing tiles if it receives an error with a particular format - see https://github.com/mapbox/tilelive.js/blob/f06db2bb9a1257c40894051e2926a97e536d988a/lib/copytask.js#L177 .