Improve error handling of posting to invalid sessions

[slilichenko]

When actively developing in ADK a server reload is often required, which by default removes all in-memory stores. The UI is not aware that the current session is no longer available on the server and attempts to POST. That POST returns 404 error, but there is no indication in the UI that the current session is "dead". Ideally, it should offer to start a new one.

[wyf7107]

Hi slilichenko, thank you for reporting this, for now if you refresh on the old session, the UI will prompt to create a new session for you.