check if source is connected before adding callback

kabuki/sources/cachedsource.py

@@ -4,6 +4,9 @@ from ..cache import Cache
 class CachedSource:
 
     def __init__(self, size=1):
+        if not self.is_connected:
+            raise RuntimeError(f"{self.pvname} is not connected, cannot attach cache")
+
         self.cache = cache = Cache(size=size)
         self._cb = self.add_callback(cache.callback_append, with_ctrlvars=False)
 
@@ -12,7 +15,3 @@ class CachedSource:
 
 
 
-#TODO: check if source is connected before adding callback
-
-
-