**Apache Airflow version**: 2.0.0a1

**What happened**:

On my dev environment I have a lot of DAG runs with failed tasks, where the *downstream* task instances then have been set to `upstream_failed`. The scheduler is now refusing to schedule more instances for such DAGs.

The scheduler looks for such task instances that are not in the `State.finished` set:

https://github.com/apache/airflow/blob/db3fe0926bb75008311eed804052c90bfa912424/airflow/jobs/scheduler_job.py#L1482-L1488

However, `State.finished` is currently not a proper inverse of *running* states, it only includes success, failed, and skipped:

https://github.com/apache/airflow/blob/db3fe0926bb75008311eed804052c90bfa912424/airflow/utils/state.py#L110-L114

What is missing is the task instance `upstream_failed` state, so currently my log is being filled (very rapidly) with:

```
[2020-10-18 13:22:18,503] {{scheduler_job.py:1658}} INFO - DAG test_dag already has 26 active runs, not queuing any more tasks
```

Those 26 'active runs' are simply dag runs that failed and have 'upstream_failed' task instances.

Either `upstream_failed` should be added to `State.finished`, or the Scheduler task instance test should explicitly take that state into account.
airflow/models/dagrun.py
"""
        Returns the task instance specified by task_id for this dag run
        :param task_id: the task id
        :type task_id: str
        :param session: Sqlalchemy ORM Session
        :type session: Session
        """
        ti = session.query(TI).filter(
            TI.dag_id == self.dag_id,
            TI.execution_date == self.execution_date,
            TI.task_id == task_id
        ).first()
        return ti
    def get_dag(self):
        """
        Returns the Dag associated with this DagRun.
        :return: DAG
        """
        if not self.dag:
            raise AirflowException("The DAG (.dag) for {} needs to be set".format(self))
        return self.dag
    @provide_session
    def get_previous_dagrun(self, state: Optional[str] = None, session: Session = None) -> Optional['DagRun']: