fix: improve error message when entity repeat doesn't exist (fixes #784)

[Anuragkrosuru]

Closes #784

Why is this the best possible solution? Were any other approaches considered?

What are the regression risks?

Does this change require updates to documentation? If so, please file an issue here and include the link below.

Before submitting this PR, please make sure you have:

• included test cases for core behavior and edge cases in tests
• run python -m unittest and verified all tests pass
• run ruff format pyxform tests and ruff check pyxform tests to lint code
• verified that any code or assets from external sources are properly credited in comments

[Anuragkrosuru]

I've created PR #792 to address this issue. The PR adds validation to check if the entity repeat exists in the survey and provides a clearer error message pointing to the entities sheet.

[lognaturel]

Thanks for your interest in this issue, @Anuragkrosuru! We're going to revisit the implementation in light of #800 so I'm going to close this for now.