### 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)

tests/api_connexion/endpoints/test_user_endpoint.py
users = self._create_users(200)
        self.session.add_all(users)
        self.session.commit()
        response = self.client.get("/api/v1/users", environ_overrides={'REMOTE_USER': "test"})
        assert response.status_code == 200
        
        assert response.json["total_entries"] == 200 + len(['test', 'test_no_permissions'])
        assert len(response.json["users"]) == 100
    def test_should_response_400_with_invalid_order_by(self):
        users = self._create_users(2)
        self.session.add_all(users)
        self.session.commit()
        response = self.client.get("/api/v1/users?order_by=myname", environ_overrides={'REMOTE_USER': "test"})
        assert response.status_code == 400
        msg = "Ordering with 'myname' is disallowed or the attribute does not exist on the model"
        assert response.json['detail'] == msg
    def test_limit_of_zero_should_return_default(self):
        users = self._create_users(200)
        self.session.add_all(users)
        self.session.commit()
        response = self.client.get("/api/v1/users?limit=0", environ_overrides={'REMOTE_USER': "test"})
        assert response.status_code == 200
        
        assert response.json["total_entries"] == 200 + len(['test', 'test_no_permissions'])
        assert len(response.json["users"]) == 100
    @conf_vars({("api", "maximum_page_limit"): "150"})
    def test_should_return_conf_max_if_req_max_above_conf(self):