Allow for further differentiation of exit codes by plugins

[stuhood]

When Pants exits with a non-zero exit code, there can be many different reasons: not all of which represent errors.

Two cases that come to mind:

• Hitting Ctrl+C
• Using ./pants run $script on a script that we expect might fail

Currently, StreamingWorkunitHandler plugins cannot detect these cases, but they're important for differentiating failure. We should likely extend the RunTracker exit code, and likely also the Goal API to allow for differentiating these cases.

[asherf]

there is already an outcome field in run tracker, I think adding values there makes sense.

[github-actions[bot]]

This issue has been open for over one year without activity and is not labeled as a bug. It has been labeled as stale to invite any further updates. If you can confirm whether the issue is still applicable to the latest version of Pants, appears in other contexts, or its priority has changed, please let us know. Please feel free to close this issue if it is no longer relevant.