Btrfs: fix error handling in make/read block group

I noticed that we will add a block group to the space info before we add it to
the block group cache rb tree, so we could potentially allocate from the block
group before it's able to be searched for.  I don't think this is too much of
a problem, the race window is microscopic, but just in case move the tree
insertion to above the space info linking.  This makes it easier to adjust the
error handling as well, so we can remove a couple of BUG_ON(ret)'s and have real
error handling setup for these scenarios.  Thanks,

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