Fix issue 1401: Raise exceptions in ClientSession

Author: KartavyaDikshit

src/mcp/client/session.py

@@ -85,6 +85,8 @@ async def __call__(
 async def _default_message_handler(
     message: RequestResponder[types.ServerRequest, types.ClientResult] | types.ServerNotification | Exception,
 ) -> None:
+    if isinstance(message, Exception):
+        raise message
     await anyio.lowlevel.checkpoint()