### Apache Airflow version

2.2.0b2 (beta snapshot)

### Operating System

Debian buster

### Versions of Apache Airflow Providers

_No response_

### Deployment

Astronomer

### Deployment details

Astro dev start

### What happened

"Already exists." error message is missing while updating user email with existing email id through API.

### What you expected to happen

"Email id already exists" error message should appear

### How to reproduce

1. Use a patch request with the below URL.
`{{url}}/users/{{username}}`
2. In payload use an exiting email id 
```
 {
        "username": "{{username}}",
        "password": "password1",
        "email": "{{exiting_email}}@example.com",
        "first_name": "{{$randomFirstName}}",
        "last_name": "{{$randomLastName}}",
        "roles":[{ "name": "Op"}]
}
```

### Anything else

_No response_

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

airflow/api_connexion/endpoints/user_endpoint.py
"""Create a new user"""
    try:
        data = user_schema.load(request.json)
    except ValidationError as e:
        raise BadRequest(detail=str(e.messages))
    security_manager = current_app.appbuilder.sm
    username = data["username"]
    email = data["email"]
    if security_manager.find_user(username=username):
        detail = f"Username `{username}` already exists. Use PATCH to update."
        raise AlreadyExists(detail=detail)
    if security_manager.find_user(email=email):
        detail = f"The email `{email}` is already taken."
        raise AlreadyExists(detail=detail)
    roles_to_add = []
    missing_role_names = []
    for role_data in data.pop("roles", ()):
        role_name = role_data["name"]
        role = security_manager.find_role(role_name)
        if role is None:
            missing_role_names.append(role_name)
        else:
            roles_to_add.append(role)
    if missing_role_names:
        detail = f"Unknown roles: {', '.join(repr(n) for n in missing_role_names)}"
        raise BadRequest(detail=detail)
    if not roles_to_add:  
        roles_to_add.append(security_manager.find_role(security_manager.auth_user_registration_role))
    user = security_manager.add_user(role=roles_to_add, **data)