Handle missing frames for sprites gracefully

If you load an addon with missing sprites on a custom character, the game outright crashes. While getting an error from this is expected, I don't think a game crash is reasonable behavior for such a case. Instead, I find it better if we just outputted an error to the console and failed gracefully, since the game handles missing sprites just fine in-game.

Character used to test this: CL_nil_v0.0.pk3