Btrfs: remove almost all of the BUG()'s from tree-log.c

There were a whole bunch and I was doing it for other things.  I haven't tested
these error paths but at the very least this is better than panicing.  I've only
left 2 BUG_ON()'s since they are logic errors and I want to replace them with a
ASSERT framework that we can compile out for production users.  Thanks,
Signed-off-by: Josef Bacik <jbacik@fusionio.com>