### 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
def test_should_respond_200(self):
        users = self._create_users(1)
        self.session.add_all(users)
        self.session.commit()
        response = self.client.get("/api/v1/users/TEST_USER1", environ_overrides={'REMOTE_USER': "test"})
        assert response.status_code == 200
        assert response.json == {
            'active': None,
            'changed_on': DEFAULT_TIME,
            'created_on': DEFAULT_TIME,
            'email': 'mytest@test1.org',
            'fail_login_count': None,
            'first_name': 'test1',
            'last_login': None,
            'last_name': 'test1',
            'login_count': None,
            'roles': [],
            'username': 'TEST_USER1',
        }
    def test_should_respond_404(self):