Fix bug with on_error links

___ test_workflow21[True-True-args1] ____

args = {'inputs': {'a': 0}, 'return_value': 2}, on_error = True, persist = True, ppf_log_config = None, tmpdir = local('/tmp/pytest-of-denolf/pytest-16/test_workflow21_True_True_args0')

    @pytest.mark.parametrize(
        "args",
        [ARG_SUCCESS, ARG_FAILURE],
    )
    @pytest.mark.parametrize("on_error", [True, False])
    @pytest.mark.parametrize("persist", [True, False])
    def test_workflow21(args, on_error, persist, ppf_log_config, tmpdir):
        """Test conditions in output nodes"""
        if persist:
            varinfo = {"root_uri": str(tmpdir)}
        else:
            varinfo = None
        graph = workflow21(on_error=on_error)
        inputs = [{"name": k, "value": v} for k, v in args["inputs"].items()]
        result1 = execute_graph(graph, inputs=inputs, varinfo=varinfo)
>       assert result1
E       assert None

ewoksppf/tests/test_ppf_workflow21.py:168: AssertionError
------------------------------------------------------------------------------------------------- Captured log call -------------------------------------------------------------------------------------------------
ERROR    pypushflow:PythonActor.py:174 [submodel] [<EwoksPythonActor> submodel:gt] Error in python actor 'submodel:gt'!
 Not running down stream actors ['Name mapper <submodel:gt - out1>']
 Exception:
"""
Traceback (most recent call last):
  File "/home/denolf/dev/ewokscore/ewokscore/task.py", line 283, in execute
    self.run()
  File "/home/denolf/dev/ewokscore/ewokscore/methodtask.py", line 18, in run
    result = method(*args, **kwargs)
  File "/home/denolf/dev/ewoksppf/ewoksppf/tests/test_ppf_workflow21.py", line 16, in raise_not_greater_than
    assert a > LIMIT
AssertionError: assert 0 > 10

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/users/denolf/.pyenv/ubuntu_20_04/versions/3.8.5/lib/python3.8/multiprocessing/pool.py", line 125, in worker
    result = (True, func(*args, **kwds))
  File "/home/denolf/dev/ewoksppf/ewoksppf/ppfrunscript.py", line 23, in run
    task.execute()
  File "/home/denolf/dev/ewokscore/ewokscore/task.py", line 289, in execute
    raise RuntimeError(f"Task '{self.label}' failed") from e
RuntimeError: Task 'submodel:gt' failed
"""