Distinguish "backend not reachable" and "conference unavailable"

[ascheman]

WRT to dukecon/dukecon#59 I pick up the RfE of @mkarg: the frontend should show separate messages depending on the backend state:

• if the backend is unreachable, it should show something like "The conference data cannot be loaded",
• if the backend does not provide the respective conference data it should show something like "The conference is not (yet) available" (as of now).