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
@provide_session
    def test_serialize(self, session):
        dagrun_model = DagRun(
            run_id="my-dag-run",
            run_type=DagRunType.MANUAL.value,
            execution_date=timezone.parse(self.default_time),
            start_date=timezone.parse(self.default_time),
            conf='{"start": "stop"}',
        )
        session.add(dagrun_model)
        session.commit()
        dagrun_model = session.query(DagRun).first()
        deserialized_dagrun = dagrun_schema.dump(dagrun_model)
        self.assertEqual(
            deserialized_dagrun,
            {
                "dag_id": None,
                "dag_run_id": "my-dag-run",
                "end_date": None,
                "state": "running",
                "execution_date": self.default_time,
                "external_trigger": True,
                "start_date": self.default_time,
                "conf": {"start": "stop"},
            },
        )
    @parameterized.expand(
        [
            (