Detect offline mode

[mkalb]

Related to #198

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[Cervator]

Before I make a separate issue to cover https://github.com/MovingBlocks/Terasology/issues/1186 would this be an appropriate existing issue? In short: When Jenkins is unavailable it should be indicated better in the UI. Currently with our server down you can run the launcher fine and it loads - it just doesn't show any available news/builds. Would be better if it clearly signaled it was unable to get info.

Unsure if the intent here differs (like "no internet available" at all)

Maybe future related note: One day we could consider GitHub releases as "valid" as files straight from Jenkins (to help harden against just this issue - Jenkins being unavailable)