Hack around missing images.

[Hoikas]

Currently. if an image is missing at export time, an irrecoverable error is raised. While this is probably, in theory, the best behavior, it poses an annoying obstacle to exchanging blend files. Instead, we should consider exporting a dynamic text map with xDynaTextBoy.py set to print "ERROR" onto it.

[dpogue]

A simpler way to accomplish the same thing would be to have an error image at a known file path (or embedded in korlib as a byte array) that could be exported

[Hoikas]

This would work well with my plan to remove PyKorlib.