Fix AttributeError on OOM

[simonhammes]

This fixes the following error in case a container uses more memory than allowed:

[ERROR] [worker_4] root runner.py:486 worker 'str' object has no attribute 'error'
Traceback (most recent call last):
  File "/opt/seatable-python-starter/runner.py", line 484, in worker
    run_python(data)
  File "/opt/seatable-python-starter/runner.py", line 437, in run_python
    output.error("Out of memory!!!")
    ^^^^^^^^^^^^
AttributeError: 'str' object has no attribute 'error'