Change Volume UUID.

Well, that describes the state after the failure (although setting the mountpoint will cause the filesystem to be remounted), but doesn't say what would cause the failure. I think trying to set another property when setting the first fails is like to fail, and we'd need code elsewhere to deal with that case anyway.

I think for the moment, the only thing to do is to report the error to a higher layer and bail. It would be nice if we could do the rename + change the mount point in a single step, but it doesn't look like `zfs(1)` allows that. (cc: @avg-I).