#9 specify conditional execution of tasks

[Zen Scient]

Allow execution of a task to be predicated on
(un)successful execution of a previous task from the
same ottm8 run. The condition should be specifiable
both in terms of whether an exception was (not) thrown
and the maximum log level of any messages logged during
execution of the task.

Note that this support will need to be built on top of
the feature to add names to tasks.