AAI-289 Add a more useful response for registration errors

[marius-mather]

Description

AAI-289: add an error response for registration requests that's easier to use in the frontend. Where possible, the response should specify which fields have errors and individual error messages for them.

Changes

• Add a RegistrationErrorResponse that is returned (with status code 400) for registration errors.
• Use RegistrationErrorResponse in registration endpoints
• Add a custom route handler so we can override FastAPI's default validation to raise the error in the format we want
• Update tests

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have added unit / integration tests that prove my fix is effective or that my feature works
• I have run all tests locally and they pass
• I have updated the documentation (if applicable)

How to Test Manually (if necessary)

Run uv run pytest

[amandazhuyilan]

Looks good - I’ll also try some errors out once this is merged!

[marius-mather]

@amandazhuyilan we still need to update the frontend to use these errors - but they should all come back in a sensible format now