tools/cpiod: Show bad CPIO magic

If the CPIO magic check fails, include the faulty magic in the exception
message to give some clues to the poor sod who needs to debug the
issue.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
tools/cpiod: mkdirp() error reporting

If mkdirp() fails, be kind to the person debugging a dubious issue and
don't be silent about it.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>