Factor out the grub error loop, apply it to decrypt-on-boot
    
    OK, we now need to work around this goddamn grub bug in *three*
    places, so let's stop copying the loop around and factor it out
    instead. The third place is encrypted installs, as they wait
    for the decryption prompt on boot.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>