Catching exceptions in Changes.ObserveContinuous()

[SamuelWeibel]

Changes.ObserveContinuous() was not properly handling two exceptional cases:

1. when the response was not successful (e.g. bad parameters, etc.) the returned observable would never complete
2. when the connection to the server was lost (e.g. when the server restarts), the failure would also not be reported to the observable; this way, the observable would never know that the observation is no longer working.

I didn't write an integration test for this yet, as I didn't try to run integration tests at all. Please feel free to add one (might require some hacking for case number two above) or ask me to write something if needed.