For deferred download with `st.download_button`, pipe errors to the script thread

[sfc-gh-dmatthews]

Checklist

• I have searched the existing issues for similar feature requests.
• I added a descriptive title and summary to this issue.

Summary

If you use deferred download with st.download_button, there should be a way to catch errors that may be raised by the deferred generation function. Currently, if the funciton encounters an error, a warning message is displayed on the frontend and isn't accessible on the backend.

Why?

When you use deferred download with st.download_button, the function which returns the download runs on its own thread so it won't block the script thread. This has two consequences:

1. There is no script context, so you can't use any Streamlit command, like st.session_state. (See Access Session State in st.download_button callable #13337).
2. You can't handle errors raised by the deferred download function.

You can put try-catch blocks in your download function to avoid an unhandled error, but you can't really do anything about it because you can't display a custom message or log something in Session State. (Maybe you could hack a solution by writing something to a temporary file on the server, but that doesn't seem ideal.)

How?

A couple different solutions:

• Have the download button return an error if encountered so it can be caught in the main script.
• Give the download thread some (possibly restricted) script context (like Access Session State in st.download_button callable #13337).
• Improve multithreading support more generally, and have some hook at the end of a script run to catch a session's child threads, like the download thread.

Additional Context

No response

[github-actions]

To help Streamlit prioritize this feature, react with a 👍 (thumbs up emoji) to the initial post.

Your vote helps us identify which enhancements matter most to our users.