fix: add an error message to say when an email is already taken

[gauthier-th]

Description

When the email is modified in the user settings and it is already taken by someone else, a generic message saying that something wrong happened, without saying that it is because the email is already taken by another user. This PR adds this error message for the email.

Screenshot (if UI-related)

To-Dos

• [x] Successful build pnpm build
• [ ] Translation keys pnpm i18n:extract
• [ ] Database migration (if required)

Issues Fixed or Closed

• Fixes #XXXX