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
result = dagrun_schema.load(serialized_dagrun)
        self.assertDictEqual(result, expected_result)
    def test_autofill_fields(self):
        """Dag_run_id and execution_date fields are autogenerated if missing"""
        serialized_dagrun = {}
        result = dagrun_schema.load(serialized_dagrun)
        self.assertDictEqual(
            result,
            {"execution_date": result["execution_date"], "run_id": result["run_id"]},
        )
class TestDagRunCollection(TestDAGRunBase):
    @provide_session
    def test_serialize(self, session):
        dagrun_model_1 = DagRun(
            run_id="my-dag-run",
            execution_date=timezone.parse(self.default_time),
            run_type=DagRunType.MANUAL.value,
            start_date=timezone.parse(self.default_time),
            conf='{"start": "stop"}',
        )
        dagrun_model_2 = DagRun(
            run_id="my-dag-run-2",
            execution_date=timezone.parse(self.default_time),
            start_date=timezone.parse(self.default_time),
            run_type=DagRunType.MANUAL.value,
        )
        dagruns = [dagrun_model_1, dagrun_model_2]
        session.add_all(dagruns)
        session.commit()
        instance = DAGRunCollection(dag_runs=dagruns, total_entries=2)