fix(accountSettings): error handling in email section DEV-1119

[magicznyleszek]

🗒️ Checklist

1. run linter locally
2. update developer docs (API, README, inline, etc.), if any
3. for user-facing doc changes create a Zulip thread at #Support Docs Updates, if any
4. draft PR with a title <type>(<scope>)<!>: <title> DEV-1234
5. assign yourself, tag PR: at least Front end and/or Back end or workflow
6. fill in the template below and delete template comments
7. review thyself: read the diff and repro the preview as written
8. open PR & confirm that CI passes & request reviewers, if needed
9. delete this section before merging

📣 Summary

Displays error in UI when changing email and API validates the proposed email as invalid.

💭 Notes

Supersedes #4224.

👀 Preview steps

To test error handling from BE, go to emailSection.component.tsx and modify handleSubmit function (comment out all the lines but setNewUserEmail(email.newEmail)).

1. ℹ️ have an account
2. go to Account Settings → Security
3. type wrong email and use "Change" button
4. 🔴 [on main] no error being displayed
5. 🟢 [on PR] error being displayed next to the button