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
deserialized_dagruns = dagrun_collection_schema.dump(instance)
        self.assertEqual(
            deserialized_dagruns,
            {
                "dag_runs": [
                    {
                        "dag_id": None,
                        "dag_run_id": "my-dag-run",
                        "end_date": None,
                        "execution_date": self.default_time,
                        "external_trigger": True,
                        "state": "running",
                        "start_date": self.default_time,
                        "conf": {"start": "stop"},
                    },
                    {
                        "dag_id": None,
                        "dag_run_id": "my-dag-run-2",
                        "end_date": None,
                        "state": "running",
                        "execution_date": self.default_time,
                        "external_trigger": True,
                        "start_date": self.default_time,
                        "conf": {},
                    },
                ],
                "total_entries": 2,
            },
        )