Rework documentation of notifications

- move notifications docs from inputs and outputs to separate page;
- listeners and TransitionNotifier documented;
  - docstrings improved, missing docstrings added;
  - documentation put to notifications page;
- inputs and outputs page edited.

Change-Id: Ib283836173a806fbec81aa07b3292e2672bf6483
Removed unused TIMED_OUT state

Change-Id: Ifca2ba5fc3e5d032ef4ab6a142b1468cde435efc
Raise error if atom asked to schedule with unknown intention

Instead of returning none from the schedule task/retry method
if a unknown atom intention is declared (this will cause later
failures) raise a execution failure when this happens to denote
that this is a problematic & exceptional case.

Change-Id: Id0e3211389e22459c5381f4a19d80d8d0baf1e25
Leave the execution_graph as none until compiled

Instead of forcing a compile when accessing the
execution_graph property just leave it as none until
compiling occurs.

It seems awkward to have the side-effect of automatically
compiling occur when a engine property is accessed.

Change-Id: I2760c369bc5926c6293199cb7e500e98d6c3eeb9
Merge "Rework documentation of notifications"
Merge "Raise error if atom asked to schedule with unknown intention"
Move taskflow.utils.misc.Failure to its own module

Failure class is important part of TaskFlow API, so it should
be more visible and accessible.

Breaking change: any client that used taskflow.utils.misc.Failure
should be updated.

Change-Id: Ib30000c9246bbcb227b34dfb0aba4d0b950bf926
Merge "Removed unused TIMED_OUT state"
Merge "Leave the execution_graph as none until compiled"
Revert "Move taskflow.utils.misc.Failure to its own module"

This reverts commit 42ca240e8157b840c298d14fbf478ae570376633 which
was a breaking change in a library consumed by other OpenStack
projects with no deprecation or backwards compatibility
considerations. It was able to merge because openstack/taskflow is
apparently not yet part of the integrated gate via the proposed
I202f4809afd689155e2cc4a00fc704fd772a0e92 change.

Change-Id: I96cf36dc317499df91e43502efc85221f8177395
Closes-Bug: #1300161