spapr: Add missing checks for NULL pointers and report failures

This fixes potential runtime crashes and two warnings from Coverity.

The new error message does not add  a prefix "qemu:" because that is
already done in function hw_error. It also starts with an uppercase
letter because that seems to be the mostly used form.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
[agraf: fix typo]
Signed-off-by: Alexander Graf <agraf@suse.de>
