Show error screens when we can't get a JWT token

[robintown]

The current behavior when we can't obtain a token from the JWT service (whether due to a connection failure or a configuration error) is to do nothing. The call will sit in an unconnected state forever. As long as we know we won't retry the JWT token fetch, we should bubble this error up to the user as an error screen.

It's going to become more important to get this done as soon as we try to lock down the JWT service to only allow creation of LiveKit rooms for users on the same homeserver deployment as the SFU. (https://github.com/element-hq/voip-internal/issues/294)