"Host does not exist"
While this is technically accurate, it doesn't offer much help to a new user. I was immediately aware after I returned to the Session screen, but I feel like a more descriptive error might prevent new users from being confused.
Perhaps offering suggestions or tips (such as "Are you sure you've entered your information correctly?", or other tips generally reported for 404'ed pages) would be helpful. The user has either mistyped their information, or perhaps the host is down, or there's a networking issue on the client side. There was never a moment in my thought process where I considered the idea that my actual host did not exist.