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
"""List of DAGRuns with metadata"""
    dag_runs: List[DagRun]
    total_entries: int
class DAGRunCollectionSchema(Schema):
    """DAGRun Collection schema"""
    dag_runs = fields.List(fields.Nested(DAGRunSchema))
    total_entries = fields.Int()
class DagRunsBatchFormSchema(Schema):
    """Schema to validate and deserialize the Form(request payload) submitted to DagRun Batch endpoint"""
    class Meta:
        """Meta"""
        datetimeformat = 'iso'
        strict = True
    page_offset = fields.Int(missing=0, min=0)
    page_limit = fields.Int(missing=100, min=1)
    dag_ids = fields.List(fields.Str(), missing=None)
    execution_date_gte = fields.DateTime(missing=None, validate=validate_istimezone)
    execution_date_lte = fields.DateTime(missing=None, validate=validate_istimezone)
    start_date_gte = fields.DateTime(missing=None, validate=validate_istimezone)
    start_date_lte = fields.DateTime(missing=None, validate=validate_istimezone)
    end_date_gte = fields.DateTime(missing=None, validate=validate_istimezone)
    end_date_lte = fields.DateTime(missing=None, validate=validate_istimezone)
dagrun_schema = DAGRunSchema()
dagrun_collection_schema = DAGRunCollectionSchema()
dagruns_batch_form_schema = DagRunsBatchFormSchema()