on_error Step Hook

A hook step to execute after the parent step if the parent step terminates abnormally in any way other than those handled by the on_abort or on_failure. This covers scenarios as broad as configuration mistakes, temporary network issues with the workers, or running longer than a timeout.

Until notifications become first-class (RFC #28, this step can be used to notify folks if their builds errored out:

- do:
  - get: ci
  - task: unit
    file: ci/unit.yml
    put: slack