Improve connection status and error reporting

[stevejpurves]

A number of things can happen during the process of connecting to a server and starting a session that may result in failures. Not all of these make it out of the API or are handled properly, we need a small pass to improve error handling at critical times.

• host unreachable
• 403 bad token on direct connect
• failures during binder launch
• failure to start a kernel - unknown kernel / bad name
• kernel dies, heartbeat fails
• server becomes unreachable after connection

in all of these conditions the javascript API should throw (not eat) errors, whilst also emitting error events -- leaving it to the caller to decide how to handle.

[stevejpurves]

this was resolved in thebe-*@0.4.3