1) Don't panic since we know exactly where this is coming from and don't
need the user to see a full back trace
2) Add docs explaining this situation a bit further
3) Cleanup the error to look like other similiar errors
In preperation for adding the ability to join a machine to an existing
cluster force the user to specify whether they expect this to me a new
cluster or an active one.
The error for not specifying the initial-cluster-state is:
```
etcd: initial cluster state unset and no wal found
```
