Misc task cleanup.

- Make some tasks less chatty by making them ConsoleTasks.
- Get rid of a local artifact wiping task. The concept of wiping
  just the local artifact cache is not coherent (what about the
  remote cache?) so you should only do that if you know what you're
  doing, in which case you should know how to do it...
- Add some safety around cleaning operations.

Auditors: pl