connlib: surface interrupted portal connection to user

[thomaseizinger]

Currently, phoenix-channel will reconnect in the background if the portal connection fails and there is no way for the client application to known that we aren't connected to the portal. This results in weird behaviour where everything appears to hang.

It also does! Without a portal connection, messages like connection intents are just buffered and never sent so we can't make new connections.

Connlib should emit some kind of event to the application to inform it that the portal connection is interrupted so that something isn't working.