Fix exception cause in agent_builder.py

Author: cool-RR

autogen/agentchat/contrib/agent_builder.py

@@ -402,7 +402,7 @@ def load(
         try:
             print(f"Loding config from {filepath}")
             cached_configs = json.load(open(filepath))
-        except FileNotFoundError:
-            raise FileNotFoundError(f"Config file {filepath} does not exist.")
+        except FileNotFoundError as e:
+            raise FileNotFoundError(f"Config file {filepath} does not exist.") from e
 
         return self.build(cached_configs=cached_configs, **kwargs)