powerpc/pseries/memhp: Log more error conditions in add path

When an add operation for multiple LMBs fails, there is currently
little indication from the kernel of what went wrong. Be a little more
verbose about error conditions in the add paths.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231114-pseries-memhp-fixes-v1-3-fb8f2bb7c557@linux.ibm.com