5770 Add load_nvlist() error handling

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Richard Elling <richard.elling@richardelling.com>
Reviewed by: Richard PALO <richard@NetBSD.org>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author: Brian Behlendorf <behlendorf1@llnl.gov>

illumos/illumos-gate@a45f1c3c5ee26f908655e7762d7e1e2aecdfefbc