Unclear error message when adding nonexistent dependency

[TheChymera]

I keep getting this error when I am adding an nonexistent dependency with, e.g.

\begin{pythontexcustomcode}[begin]{py}
pytex.add_dependencies('some/file/that/does/not.exist')
\end{pythontexcustomcode}

:

This is PythonTeX 0.16
Traceback (most recent call last):
  File "/usr/lib/python-exec/python3.6/pythontex.py", line 62, in <module>
    pythontex.main()
  File "/usr/lib64/python3.6/site-packages/pythontex3.py", line 2716, in main
    do_multiprocessing(data, temp_data, old_data, engine_dict)
  File "/usr/lib64/python3.6/site-packages/pythontex3.py", line 1415, in do_multiprocessing
    if val[0] > start_time:
TypeError: '>' not supported between instances of 'NoneType' and 'float'

And I always keep spending a lot of time trying to figure out what's wrong. I'm thinking this is the best place to document this, not least of all because it would be cool to improve the error message in the next release :)