Switch empty dfk() error from ConfigurationError to NoDataFlowKernelE…

…rror (#2890)

This is in support of
#2873 (comment)

parsl/dataflow/dflow.py

@@ -1438,5 +1438,5 @@ def wait_for_current_tasks(cls) -> None:
     def dfk(cls) -> DataFlowKernel:
         """Return the currently-loaded DataFlowKernel."""
         if cls._dfk is None:
-            raise ConfigurationError('Must first load config')
+            raise NoDataFlowKernelError('Must first load config')
         return cls._dfk