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

airflow/api_connexion/schemas/dag_run_schema.py
"""Schema for DAGRun"""
    class Meta:
        """Meta"""
        model = DagRun
        dateformat = "iso"
    run_id = auto_field(data_key='dag_run_id')
    dag_id = auto_field(dump_only=True)
    execution_date = auto_field(validate=validate_istimezone)
    start_date = auto_field(dump_only=True)
    end_date = auto_field(dump_only=True)
    state = DagStateField(dump_only=True)
    external_trigger = auto_field(default=True, dump_only=True)
    conf = ConfObject()
    @pre_load
    def autogenerate(self, data, **kwargs):
        """Auto generate run_id and execution_date if they are not loaded"""
        if "execution_date" not in data.keys():
            data["execution_date"] = str(timezone.utcnow())
        if "dag_run_id" not in data.keys():
            data["dag_run_id"] = DagRun.generate_run_id(
                DagRunType.MANUAL, timezone.parse(data["execution_date"])
            )
        return data
class DAGRunCollection(NamedTuple):