Missing required 'file's should fail gbuild before bringing image up

[gdm85]

Please check for failure when doing the sha256sum of the 'files'.

Possibly at https://github.com/devrandom/gitian-builder/blob/master/bin/gbuild#L183

Otherwise failure will happen anyway in the descriptor's script when it's trying to use them, but user will have spoiled the time to bring up a new image