Conditional branching

**What would you like to be added**:

Several people at Meetups have asked about if then else branching within plans. Plans currently handle if then at the task level based on success of the previous task but I'm not sure how/if we handle else.

**Why is this needed**:

Within a plan you may want to have a task run on failure of a previous task. This could probably be achieved by adding an OnFailure attribute to a task, which would be another task.

Start with the KEP process linked in the issue and review how KUDO plans currently represent task-level branching after success. Define the expected behavior for failure-based branching, including how an else path should be specified, before identifying implementation and test locations; done means the behavior is agreed through the KEP process and covered by tests.

Written by the indexing model from the issue text.