If nobody in the chat room can know that you lost connectivity, that means you can't do real time chat and might as well use email instead.

Note that a similar thing happens with any two-way conversation; if an immediate reply is expected and there's no reply, it's not a conversation anymore. (For more possible reasons, though.)

Disappearances can't have good error messages if you want plausible deniability. This will make any UI much less user-friendly.