[BUG] Internal error code exposed when email or username already exists during user creation

Description:

When creating a new user from the Admin panel, Rocket.Chat shows an internal error key ([error-field-unavailable]) instead of a user-friendly message if the email or username already exists.

This exposes an internal error identifier to the UI and causes confusion for administrators.

Steps to reproduce:

1. Go to 'Administration → Users'
2. Click on 'New user'
3. Enter an email address that already exists
4. OR enter a username that already exists
5. Submit the form

Expected behavior:

A clear, human-readable validation message should be shown, for example:

• “Email address already exists”
• “Username already exists”

Actual behavior:

The UI displays an internal error message such as:

miraS is already in use :( [error-field-unavailable]
mirasen@gmail.com is already in use :( [error-field-unavailable]

(See attached screenshots)

Server Setup Information:

• Version of Rocket.Chat Server: latest (Docker)
• License Type: Starter
• Number of Users: 2
• Operating System: Windows
• Deployment Method: Docker Compose
• Number of Running Instances: 1
• DB Replicaset Oplog: Enabled (default docker setup)
• NodeJS Version: Bundled with Docker image
• MongoDB Version: Docker image default

Client Setup Information

• Desktop App or Browser Version: Chrome (latest)
• Operating System: Windows

Additional context

This appears to be an error-mapping or UI validation issue where backend error identifiers are directly surfaced in the UI instead of translated messages.

This impacts admin UX during user creation.

Relevant logs:

No crash logs observed.
Issue appears to be related to frontend error handling / message mapping.

Start in the Administration → Users → New user flow and reproduce the duplicate-email and duplicate-username cases described in the issue. Trace the frontend error mapping that displays the backend identifier, then verify that each case shows a clear human-readable validation message instead of [error-field-unavailable].