Fix choice error behavior

[natefaubion]

This restores the error behavior of choice. Previously it was left associative, so the empty node was the first thing processed (and discarded). By making it right associative, it's the last thing processed, making it identical to oneOf, which means you can often get a No alternative error which isn't helpful. This restores the previous error behavior.

I don't think this requires a changelist item.

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Checklist:

• Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation in the README and/or documentation directory
• Added a test for the contribution (if applicable)