Error out if trying to extract images from a file without any

src/lib.rs

@@ -39,9 +39,11 @@ pub fn extract_first_image(music_filename: &Path, image_filename: &Path) -> Resu
             },
             Err(e) => return Err(format!("Couldn't load image: {}", e).into()),
         };
-    }
 
-    Ok(())
+        Ok(())
+    } else {
+        Err("No image found in music file".into())
+    }
 }
 
 pub fn remove_images(music_filename: &Path) -> Result<(), Box<Error>> {

tests/test_processing.rs

@@ -163,3 +163,17 @@ fn test_overwriting_an_existing_image() {
 
     assert!(image.exists());
 }
+
+#[test]
+fn test_extracting_an_image_with_no_pictures() {
+    let song  = Fixture::copy("attempt_1_no_image.mp3");
+    let image = Fixture::blank("attempt_1.jpg");
+
+    let tag = read_tag(&song);
+    assert!(tag.pictures().count() == 0);
+    assert!(!image.exists());
+
+    assert!(extract_first_image(&song, &image).is_err());
+
+    assert!(!image.exists());
+}