Tracker: show task advancement label. Refs #2817
Tracker: fixed findFailedSubtasks() method and trackerError template. Refs #2817
Observer ptrack guideline for error reporting. Refs #2817

" When a task has non-zero exit code the message can specify the error
  reason.  If the message is empty, the reason should be provided by a
  more specific sub-task. "
NethPlatform: moved ptrack invocation into Process.
Tracker: define notification templates in XHTML view. Refs #2818
System/Process: experimental on() method. Refs #2817

The on() method sets what to do when the detached process
exits. Possible actions:

* show a notification

* execute a query request (optionally freezing the UI)