Enhance multiprocess manager's log data

[przlada]

Summary

This PR proposes to extend the multiprocess manager's "Child process died" log with additional information, specifically the exit code and the name of the signal that terminated the process, similarly as gunicorn is reporting. This change will provide more context for troubleshooting issues related to unexpected process termination, particularly in scenarios such as processes being terminated due to "out of memory" errors.

The additional information will be added as extra values in the log.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.