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
#
#
import unittest
from dateutil.parser import parse
from parameterized import parameterized
from airflow.api_connexion.schemas.dag_run_schema import (
    DAGRunCollection,
    dagrun_collection_schema,
    dagrun_schema,
)
from airflow.models import DagRun
from airflow.utils import timezone
from airflow.utils.session import provide_session
from airflow.utils.types import DagRunType
from tests.test_utils.db import clear_db_runs
DEFAULT_TIME = "2020-06-09T13:59:56.336000+00:00"
class TestDAGRunBase(unittest.TestCase):
    def setUp(self) -> None:
        clear_db_runs()
        self.default_time = DEFAULT_TIME
    def tearDown(self) -> None:
        clear_db_runs()
class TestDAGRunSchema(TestDAGRunBase):