Fix wrong instance variable name in error message

When not enough data is written with a CachedDataSink the error message will trigger an error because it uses a wrong instance variable name.

This patch fixes this.

beat/backend/python/data.py

@@ -968,7 +968,7 @@ class CachedDataSink(DataSink):
                     message = "No data written"
                 else:
                     message = "No enough data written: last written {} vs end {}".format(
-                        self.last_data_index, self.end_index
+                        self.last_written_data_index, self.end_index
                     )
 
                 logger.warning("Removing cache files: {}".format(message))