channel_db: raise specific exception if database is not loaded when we try to find a route

electrum/tests/test_lnpeer.py

@@ -55,6 +55,7 @@ class MockNetwork:
         self.config = simple_config.SimpleConfig(user_config, read_user_dir_function=lambda: user_dir)
         self.asyncio_loop = asyncio.get_event_loop()
         self.channel_db = ChannelDB(self)
+        self.channel_db.data_loaded.set()
         self.path_finder = LNPathFinder(self.channel_db)
         self.tx_queue = tx_queue