better errors for loading sprites

src/sprite_provider.js

@@ -9,6 +9,9 @@ export class SpriteProvider {
 
     url(name) {
         const sprite = this.sprites.find((x) => x.name === name);
+        if (!sprite) {
+            throw new Error(`Sprite with name '${name}' does not exist`);
+        }
         return `data:${sprite.mime};base64,${sprite.bytes.toBase64()}`;
     }
 }