Add an error enum to test/codec.rs

tests/codec.rs

@@ -7,6 +7,18 @@ use std::io::BufWriter;
 use std::path::Path;
 use std::{fs::File, io::Read};
 
+#[derive(Debug)]
+enum TestError {
+    SetupFailure, // an error during test initialization (missing file, no write permission for output)
+    CodecFailure(DecodeError),
+}
+
+impl From<DecodeError> for TestError {
+    fn from(err: DecodeError) -> Self {
+        Self::CodecFailure(err)
+    }
+}
+
 #[test]
 fn main() -> Result<(), DecodeError> {
     let file = File::open("qoi_test_images/dice.qoi").unwrap();