fix infinite loop due to panic from bare .unwrap of a Result in create_digests intrinsic

[tdyas]

While investigating #21709, the investigation was obscured by the fact that the issue manifested as a panic in the create_digests intrinsic. The error was in the Err variant of a Result which the create_digests blindly called .unwrap() on, which of course panics by design.

This panic resulted in the engine retrying the build graph node at issue and hitting the panic again ... and again ... and again ...

The engine should really stop if it encounters a panic from a node computation, but in the meantime, the create_digests should just return errors in Result instances instead of panicking.

[tdyas]

fyi @cburroughs

[huonw]

Makes sense, good find.

[WorkerPants]

I tried to automatically cherry-pick this change back to each relevant milestone, so that it is available in those older releases of Pants.

❌ 2.24.x

I was unable to cherry-pick this PR to 2.24.x, likely due to merge-conflicts.

Steps to Cherry-Pick locally

To resolve:

1. (Ensure your git working directory is clean)
2. Run the following script to reproduce the merge-conflicts:

   git fetch https://github.com/pantsbuild/pants main \
     && git fetch https://github.com/pantsbuild/pants 2.24.x \
     && git checkout -b cherry-pick-21746-to-2.24.x FETCH_HEAD \
     && git cherry-pick 52e3797a2fb98ae3381422a9ee18f3e5b25878b7

3. Fix the merge conflicts and commit the changes
4. Run build-support/cherry_pick/make_pr.sh "21746" "2.24.x"

Please note that I cannot re-run CI if a job fails. Please work with your PR approver(s) to re-run CI if necessary.

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When you're done manually cherry-picking, please remove the needs-cherrypick label on this PR.

Thanks again for your contributions!

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