on_abort: perform a step when another aborts

Any step can have on_abort tacked onto it, whose value is a second step to execute only if the parent step aborts.

on_abort: step

The step to execute when the parent step aborts. If the attached step succeeds, the entire step is still aborted.

The following will perform the attached task only if the first one aborts:

plan:
- get: foo
- task: unit
  file: foo/unit.yml
  on_abort:
    task: alert
    file: foo/alert.yml