Handle invalid assets gracefully

[zuntrax]

As shown in #972, openage crashes when it encounters invalid sound files. In this case, a field in the header struct had the wrong size, but i can imagine a bunch of other fails like corrupted bistreams or broken graphics taking down the engine. We should be robust against all the nastyness that might be hidden inside the assets. Crashing during a game would be inacceptable, playing silence or rendering a red X would be nice. We should also have proper logging and a way to nicely test asset packs for broken files. Last but not least, we should fuzz our current implementation and have integration tests with corrupt assets in kevin.
Ideas, anybody?

[TheJJ]

With the new core engine and nyan as mod provider, we just have to think of this. Failing sounds should just be missing (+ a one-time warning), failing textures should be pink squares (+ the warning), failing nyan files will abort loading the mod.