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
#
#
#
import json
from typing import List, NamedTuple
from marshmallow import fields, pre_load
from marshmallow.schema import Schema
from marshmallow_sqlalchemy import SQLAlchemySchema, auto_field
from airflow.api_connexion.parameters import validate_istimezone
from airflow.api_connexion.schemas.enum_schemas import DagStateField
from airflow.models.dagrun import DagRun
from airflow.utils import timezone
from airflow.utils.types import DagRunType
class ConfObject(fields.Field):
    """The conf field"""
    def _serialize(self, value, attr, obj, **kwargs):
        if not value:
            return {}
        return json.loads(value) if isinstance(value, str) else value
    def _deserialize(self, value, attr, data, **kwargs):
        if isinstance(value, str):
            return json.loads(value)
        return value
class DAGRunSchema(SQLAlchemySchema):