**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
"""
        Verifies the DagRun by checking for removed tasks or tasks that are not in the
        database yet. It will set state to removed or add the task if required.
        :param session: Sqlalchemy ORM Session
        :type session: Session
        """
        dag = self.get_dag()
        tis = self.get_task_instances(session=session)
        
        task_ids = set()
        for ti in tis:
            task_instance_mutation_hook(ti)
            task_ids.add(ti.task_id)
            task = None
            try:
                task = dag.get_task(ti.task_id)
            except AirflowException:
                if ti.state == State.REMOVED:
                    pass  
                elif self.state is not State.RUNNING and not dag.partial:
                    self.log.warning("Failed to get task '%s' for dag '%s'. "
                                     "Marking it as removed.", ti, dag)
                    Stats.incr(
                        "task_removed_from_dag.{}".format(dag.dag_id), 1, 1)