fix: return better errors for username update issues (AAI-474)

[marius-mather]

Description

AAI-474: need to return better error messages that the frontend can display to give the user info on what has gone wrong

Changes

• Check error from Auth0 when updating username - return specific error messages to the frontend where possible.
• Unit test

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)
• For any new secrets, I have updated the shared spreadsheet and the GitHub Secrets.

How to Test Manually (if necessary)

Run uv run pytest

[amandazhuyilan]

Looks good - no complaints found