powerpc/pseries: Cleanup VPA registration and deregistration errors

Make the VPA, SLB shadow and DTL registration and deregistration
functions print consistent messages on error. I needed the firmware
error code while chasing a kexec bug but we weren't printing it.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>