This fix ensures that if there are specific module errors (in our case
the python interpreter was not found) then command and shell returns a
proper error.
It also fixes a few other imperfections that we noticed during
troubleshooting:
- Return the real RC if it were available
- Improve a dictionary evaluation using .get()
- Return an RC of -1 if it is unknown (instead of returning 0)
This fixes#18846
Dag Wieers
Brian Coca
Matt Clay