I had the same issue, when I tried to connect with wrong hostname, or url. Are you sure your host is correct in the url? Isn't it behind firewall? Try to  nslookup your host. 
For invalid token or cluster or invalid cluster state (not running) it returns descriptive error messages for me.
Also make sure you use the proper pyspark version which is the same as your spark version of DBR used by your cluster.