### 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
@parameterized.expand(
        [
            ("/api/v1/users?limit=1", ["test"]),
            ("/api/v1/users?limit=2", ["test", "test_no_permissions"]),
            (
                "/api/v1/users?offset=5",
                [
                    "TEST_USER4",
                    "TEST_USER5",
                    "TEST_USER6",
                    "TEST_USER7",
                    "TEST_USER8",
                    "TEST_USER9",
                    "TEST_USER10",
                ],
            ),
            (
                "/api/v1/users?offset=0",
                [
                    "test",
                    "test_no_permissions",