better error handling for Sprite

src/sprite.rs

@@ -1,6 +1,8 @@
 use crate::{optional_data_line, AnimationFrames, Image, Position};
 use crate::image::animation_frames_from_str;
 
+use std::fmt;
+
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct Sprite {
     pub id: String,
@@ -51,14 +53,14 @@ impl Sprite {
     }
 }
 
-#[derive(Debug)]
-pub struct SpriteMissingRoomPosition;
-// todo "malformed sprite ID" or something
-
 impl Sprite {
-    pub(crate) fn from(string: String) -> Result<Sprite, SpriteMissingRoomPosition> {
+    pub fn from(string: String) -> Result<Sprite, crate::Error> {
         let mut lines: Vec<&str> = string.lines().collect();
 
+        if lines.is_empty() || !lines[0].starts_with("SPR ") {
+            return Err(crate::Error::Sprite);
+        }
+
         let id = lines[0].replace("SPR ", "");
         let mut name = None;
         let mut dialogue_id: Option<String> = None;
@@ -80,13 +82,13 @@ impl Sprite {
                 room_id = Some(room_position[0].to_string());
 
                 if room_position.len() < 2 {
-                    return Err(SpriteMissingRoomPosition);
+                    return Err(crate::Error::Sprite);
                 }
 
                 if let Ok(pos) = Position::from_str(room_position[1]) {
                     position = Some(pos);
                 } else {
-                    return Err(SpriteMissingRoomPosition);
+                    return Err(crate::Error::Sprite);
                 }
             } else if last_line.starts_with("COL") {
                 colour_id = Some(last_line.replace("COL ", "").parse().unwrap());
@@ -117,9 +119,10 @@ impl Sprite {
     }
 }
 
-impl ToString for Sprite {
+impl fmt::Display for Sprite {
-    fn to_string(&self) -> String {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        format!(
+        write!(
+            f,
             "SPR {}\n{}{}{}{}{}{}",
             self.id,
             self.animation_frames.to_string(),