libbe(3): More error handling bits

be_add_child functionality gets split out into separate places as a bonus.
A lot of places here we'll gloss over libzfs errors, because they shouldn't
be happening given the conditions that we're operating under. "Unknown
error" is what I'm intending to use for the moment to indicate an
exceptional circumstance- exceptional enough that we can't tell the consumer
did because we're not so certain that they did anything.