pseries: Clean up error handling in spapr_validate_node_memory()

pseries: Clean up error handling in spapr_validate_node_memory()

Use error_setg() and return an error, rather than using an explicit exit().

Also improve messages, and be more explicit about which constraint failed.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Markus Armbruster <armbru@redhat.com>