is_transparent returns 1 for every unknown block because that's an easy way to
make them ignore lighting, however, this makes cave render useless with a map
containing unknown ores.
The definition is_known_transparent checks whether a block is transparent and is known.
This is a workaround, a proper fix would be to have unknown blocks be neither transparent nor
solid. Or make them both. But don't prefer one of the two.
Also removed some code that I accidentially left in.
Also added a traceback printing decorator around get_chunk() because the
C code can potentially swallow those exceptions.
