### 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
if "username" in masked_data:
            raise BadRequest("Cannot update fields: 'username'")
        data = masked_data
    else:
        data.pop("username", None)
    if "roles" in data:
        roles_to_update = []
        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_update.append(role)
        if missing_role_names:
            detail = f"Unknown roles: {', '.join(repr(n) for n in missing_role_names)}"
            raise BadRequest(detail=detail)
    else:
        roles_to_update = None  
    if "password" in data:
        user.password = generate_password_hash(data.pop("password"))
    if roles_to_update is not None:
        user.roles = roles_to_update
    for key, value in data.items():
        setattr(user, key, value)
    security_manager.update_user(user)
    return user_schema.dump(user)
@security.requires_access([(permissions.ACTION_CAN_DELETE, permissions.RESOURCE_USER)])
def delete_user(username):