bectl: Improve error message when libbe_init() fails

[gallatin]

Summary

I spent a long time trying to recover a remote machine where it was booting into a bad BE. Using a rather old rescue image already on the box, I encountered the error libbe_init(\"%s\") failed. I stupidly assumed the rescue image was too old to be compatible, when the problem was really that I was booted from a UFS root on the rescue image, so the tool could not deduce the rootfs I wanted to operate on. An error message like the one in this change would have prompted me that I needed to specify a ZFS rootfs in this scenario, and would have saved me hours.

[imp]

Seems legit

[kevans]

I'm OK with this, but I note that the -r option isn't documented at all because it wasn't actually supposed to be generally useful. We should probably fix that so that this message makes sense, since it *is* generally useful (for precisely scenarios like this).