make-disk-image.nix: Handle cptofs error explicitly
make-disk-image.nix: Handle cptofs error explicitly

Because the copy process inside the VM does not reliably
give "No space" error message leaving the user wondering what
went wrong:

unable to create directory /mnt/0000fe01///nix/store/yknzxx7w2ck9p30k81gpi5yfjlrq41lr-libsecret-0.18.7/share/locale/ro: Success
[    5.462365] reboot: Restarting system
error processing entry /build/root/nix/store/yknzxx7w2ck9p30k81gpi5yfjlrq41lr-libsecret-0.18.7/share/locale/ro, aborting
error processing entry /build/root/nix/store/yknzxx7w2ck9p30k81gpi5yfjlrq41lr-libsecret-0.18.7/share/locale, aborting
error processing entry /build/root/nix/store/yknzxx7w2ck9p30k81gpi5yfjlrq41lr-libsecret-0.18.7/share, aborting
error processing entry /build/root/nix/store/yknzxx7w2ck9p30k81gpi5yfjlrq41lr-libsecret-0.18.7, aborting
error processing entry /build/root/nix/store, aborting
error processing entry /build/root/nix, aborting
builder for '/nix/store/fsdvqxq92iai7f3w8wcsncgfwag7cj2l-libvirtd-ssh-image.drv' failed with exit code 228

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