This started life as a dev utility so I was only printing the message. Now that it's a `#[test]`, I'll have it panic on the error path.
I'm trying to minimize the number of panics even in test code. Some are going to stay because I'm lazy and it makes no difference for a test.
main.rs
The codec library doesn't use any external dependencies. The PNG crate is only to load some source material for testing the codec. Turning the test driver into a real test lets me turn the PNG dependency into a dev-dependency.
