fix: return proper exception for email in use

[KrissDrawing]

This PR addresses #770

• [x] Addresses the issue in full
• [ ] Addresses only certain aspects of the issue

Description

Returns 409 instead of 400 error for email in use (frontend already checks for 409 error code, and displays different message)

How Can This Be Tested/Reviewed?

In http://localhost:3001/users create user with already existing email (for example: admin@example.com). Error should display expected message

Author Checklist:

• [ ] Added QA notes to the issue with applicable URLs
• [ ] Reviewed in a desktop view
• [ ] Reviewed in a mobile view
• [ ] Reviewed considering accessibility
• [ ] Added tests covering the changes
• [ ] Made corresponding changes to the documentation
• [ ] Ran yarn generate:client and/or created a migration when required

Review Process:

• Read and understand the issue
• Ensure the author has added QA notes
• Review the code itself from a style point of view
• Pull the changes down locally and test that the acceptance criteria is met
• Either (1) explicitly ask a clarifying question, (2) request changes, or (3) approve the PR, even if there are very small remaining changes, if you don't need to re-review after the updates

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