feat: add channel playlists

- add tests for channel videos
- small model refactor (rename Channel to ChannelTag)

src/serializer/mod.rs

@@ -41,6 +41,25 @@ where
     }
 }
 
+impl<T> MapResult<T>
+where
+    T: Default,
+{
+    pub fn error(msg: String) -> Self {
+        Self {
+            c: T::default(),
+            warnings: vec![msg],
+        }
+    }
+
+    pub fn ok(c: T) -> Self {
+        Self {
+            c,
+            warnings: Vec::new(),
+        }
+    }
+}
+
 /// Deserialization method that consumes anything and returns an empty value.
 /// Intended to be used for a wildcard enum option.
 ///