bectl: fail if an invalid sort key is used

[kevans]

Summary

Otherwise we would presumably sort based on garbage. This assumes that
the set of properties fetched for each BE is consistent, thus the
assertion on subsequent nvlist lookups for the sort key.

PR: 266496, 289233
MFC after: 3 days

[tamelingdaniel_gmail.com]

@kevans can you have a look at your patch in bug 266496? (link)
I think nvlist_lookup_string can also return non-zero if one uses "origin" as key and the boot environment doesn't have an origin data set.

[kevans]

In D52498#1199266, @tamelingdaniel_gmail.com wrote:

@kevans can you have a look at your patch in bug 266496? (link)
I think nvlist_lookup_string can also return non-zero if one uses "origin" as key and the boot environment doesn't have an origin data set.

You know, I *thought* something about this sounded familiar. *sigh*. That's a fair point re: origin, let me revisit that PR and see what we can do.

[kevans]

Account for origin not existing, use an empty string; add tests