error if unconnected source is added to cacher

cacher.py

@@ -9,6 +9,8 @@ class Cacher:
 
     def add_source(self, src, size=100):
         name = src.name
+        if not src.is_connected:
+            raise RuntimeError(f"source NOT connected: {name}")
         cache = self.add_cache(name, size=size)
         src.add_callback(cache.callback_append)
         return cache