refactor: use Result with a unit type over Option

this helps convey intent, and makes it more consistent

src/main.rs

@@ -155,7 +155,7 @@ impl State {
     }
 
     fn save_image(&mut self) {
-        self.error = utils::save_image(self.image.as_ref());
+        self.error = utils::save_image(self.image.as_ref()).err();
     }
 
     #[must_use]

src/utils.rs

@@ -105,19 +105,18 @@ pub fn pick_image() -> Result<PathBuf, String> {
     Ok(path)
 }
 
-#[must_use]
-pub fn save_image(image: Option<&RgbaImage>) -> Option<String> {
+pub fn save_image(image: Option<&RgbaImage>) -> Result<(), String> {
     let Some(image) = image else {
-        return Some("no image to save".into());
+        return Err("no image to save".into());
     };
 
     let Some(path) = FileDialog::new().save_file() else {
-        return Some("no path to save provided".into());
+        return Err("no path to save provided".into());
     };
 
     let mut file = match fs::File::create(&path) {
         Ok(f) => f,
-        Err(e) => return Some(format!("failed to create file '{}': {e}", path.display())),
+        Err(e) => return Err(format!("failed to create file '{}': {e}", path.display())),
     };
 
     match path.extension().map(OsStr::to_string_lossy).as_deref() {
@@ -132,20 +131,20 @@ pub fn save_image(image: Option<&RgbaImage>) -> Option<String> {
                 match encoder.encode::<u8, u8>(&rgb_image, rgb_image.width(), rgb_image.height()) {
                     Ok(j) => j,
                     Err(e) => {
-                        return Some(format!("failed to encode jxl '{}': {e}", path.display()));
+                        return Err(format!("failed to encode jxl '{}': {e}", path.display()));
                     }
                 };
 
             if let Err(e) = file.write(&jxl.data) {
-                return Some(format!("failed to write jxl to '{}': {e}", path.display()));
+                return Err(format!("failed to write jxl to '{}': {e}", path.display()));
             };
         }
         _ => {
             if let Err(e) = image.save(&path) {
-                return Some(format!("failed to save '{}': {e}", path.display()));
+                return Err(format!("failed to save '{}': {e}", path.display()));
             };
         }
     };
 
-    None
+    Ok(())
 }