feat: support removing watch status when null

A user can choose to remove a show from being in their media list completely, by setting the watch status to null

src/libs/errors/TitleNotFound.ts

@@ -0,0 +1,5 @@
+export class AnilistTitleNotFoundError extends Error {
+  constructor() {
+    super("Title not found in Anilist");
+  }
+}