Triggering a dagrun with an invalid execution date causes a ParserError in the REST API

**Apache Airflow version**: 2.0

**What happened**:
When you trigger a dagrun with an invalid execution date, it returns an HTML response showing the error

**What you expected to happen**:

I expected a JSON response showing that the execution date is invalid

**How to reproduce it**:

1. Start airflow webserver and scheduler
2. Make a post request to this endpoint `http://localhost:28080/api/v1/dags/example_bash_operator/dagRuns` with this request body `{"execution_date": "mydate"}`
3. This will return an HTML page instead of JSON

tests/api_connexion/schemas/test_dag_run_schema.py
{"dag_run_id": "my-dag-run", "execution_date": DEFAULT_TIME},
                {"run_id": "my-dag-run", "execution_date": parse(DEFAULT_TIME)},
            ),
            (
                {
                    "dag_run_id": "my-dag-run",
                    "execution_date": DEFAULT_TIME,
                    "conf": {"start": "stop"},
                },
                {
                    "run_id": "my-dag-run",
                    "execution_date": parse(DEFAULT_TIME),
                    "conf": {"start": "stop"},
                },
            ),
            (
                {
                    "dag_run_id": "my-dag-run",
                    "execution_date": DEFAULT_TIME,
                    "conf": '{"start": "stop"}',
                },
                {
                    "run_id": "my-dag-run",
                    "execution_date": parse(DEFAULT_TIME),
                    "conf": {"start": "stop"},
                },
            ),
        ]
    )
    def test_deserialize(self, serialized_dagrun, expected_result):