Better error message for event deletion when event has attendances

[echang594]

Info

Closes #199.

Description

What changes did you make? List all distinct problems that this PR addresses. Explain any relevant motivation or context.

Throw error when trying to delete event with attendances.

Changes

• Also fixed tests that assumed repository query results are always ordered correctly.

Type of Change

• [x] Patch (non-breaking change/bugfix)
• [ ] Minor (non-breaking change which adds functionality)
• [ ] Major (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation (A change to a README/description)
• [ ] Continuous Integration/DevOps Change (Related to deployment steps, continuous integration workflows, linting, etc.)
• [ ] Other: (Fill In)

If you've selected Patch, Minor, or Major as your change type, make sure to bump the version before merging in package.json!

Testing

I have tested that my changes fully resolve the linked issue ...

• [x] locally.
• [ ] on the testing API/testing database.
• [x] with appropriate Postman routes. Screenshots are included below.

Checklist

• [x] I have performed a self-review of my own code.
• [x] I have followed the style guidelines of this project.
• [x] I have appropriately edited the API version in the package.json file.
• [x] My changes produce no new warnings.

Screenshots

The event uuid above is one that has attendances.

[github-actions[bot]]

Thanks for contributing! If you've made changes to the API's functionality, please make sure to bump the package version—see this guide to semantic versioning for details—and document those changes as appropriate.