Extend Error Notifications to REST API actions

[bturi]

Description

Extend DAG's email_on_failure feature to the case when a Task/DAG state is modified via Airflow's REST API endpoints such as updateTaskInstancesState.

Use case / motivation
Suppose a Task state is set to 'failed' via the official REST API. It would be convenient if the email_on_failure setting of the respective DAG would also apply here.

[boring-cyborg[bot]]

Thanks for opening your first issue here! Be sure to follow the issue template!