### 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
user = create_user(
        configured_app,
        username=autoclean_username,
        email="another_user@example.com",
        role_name="TestNoPermissions",
    )
    assert user, f"failed to create user '{autoclean_username} <another_user@example.com>'"
    return user
@pytest.fixture()
def user_with_same_email(configured_app, autoclean_email):
    user = create_user(
        configured_app,
        username="another_user",
        email=autoclean_email,
        role_name="TestNoPermissions",
    )
    assert user, f"failed to create user 'another_user <{autoclean_email}>'"
    return user
@pytest.fixture()
def user_different(configured_app):
    username = "another_user"
    email = "another_user@example.com"
    _delete_user(username=username, email=email)
    user = create_user(configured_app, username=username, email=email, role_name="TestNoPermissions")
    assert user, "failed to create user 'another_user <another_user@example.com>'"
    yield user
    _delete_user(username=username, email=email)
@pytest.fixture()
def autoclean_user_payload(autoclean_username, autoclean_email):