Add default cloud texture

[llbit]

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If Chunky fails to load a cloud texture it will use an empty cloud texture, preventing clouds from rendering. It would be better to provide a default cloud texture in this case.

[leMaik]

While you're at it, could you please add an option to kill chunky with a non-zero exit code if loading the cloud texture fails instead of using default values silently (as in #398). This would make using Chunky from scripts more predictable and help handling errors.