s390/kexec_file: print some more error messages

Be kind and give some more information on what went wrong.

Signed-off-by: Philipp Rudo <prudo@redhat.com>
Link: https://lore.kernel.org/r/20211208130741.5821-2-prudo@redhat.com


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>