A fun way to segfault PySoy: try to load a texture that doesn't exist!

Of course that shouldn't happen, an appropriate "File Not Found" error be raised but texture loading was rushed, this was glossed over, and right now we have a quick an easy way to crash PySoy.

Both Genie and Python's c-api have standard, well-documented ways to raise errors. Go implement them for cases where the file doesn't exist or isn't a valid, complete, and error-free PNG or SVG/SGK.

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