fix(#26): gracefully quit on auth failures in hstorm

[gormaniac]

This PR does two things to gracefully exit on auth failures in hstorm:

1. Changes the HttpCortex.login method. It moves the original login functionality to a new method, _login, which the new login method calls. It also adds an optional close argument to the new login method, that defaults to False. When True, HttpCortex.stop is called when _login raises an exception, then login reraises that exception. This new argument is set to True in the HttpCortex.__aenter__ method's login call.
2. A try/except is added to hstorm.main around the HttpCortex call and catches HttpCortexLoginError errors. This way only the error message is printed upon a login error.