Storage device expansion "silently" fails on degraded vdev

When a vdev is degraded or faulted, we refuse to expand it when doing
online -e. However, we also don't actually cause the online command
to fail, even though the disk didn't expand. This is confusing and
misleading, and can result in violated expectations.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes 14145