Root table's tablet assignment failure on startup

[Josh Elser]

I'm not sure if I did something bad the last time I stopped my 1.6.0 
instance, but I figure I'll mention it here first.

Started up a 1.6.0 instance that I had before. Root table didn't come 
online because it couldn't load its tablet because the 
/accumulo/${instance_id}/root_tablet/dir node in ZK doesn't exist.

I didn't want to just wipe it in case this is an actual problem. Thoughts?