Btrfs-progs: return an error if we can't find an fs root

Btrfs-progs: return an error if we can't find an fs root

Instead of doing a BUG_ON() if we fail to find the last fs root just return
an error so the callers can deal with it how they like.  Also we need to
actually return an error if we can't find the latest root so that the error
handling works.  With this btrfsck was able to deal with a file system that
was missing a root item but still had extents that referred back to the
root.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>