Identify source of the exception in a task trace

[jodzga]

In ParSeq exceptions are automatically propagated. It means that within the trace there are typically many failed tasks with the same exception.
It would be very useful to identify which task was a source of the exception to help troubleshooting problems.

[extesy]

@jodzga Is this task covered by #115 or does it mean something else?

[jodzga]

It means something else although it is related.
When one of the tasks fails typically the Exception will be propagated to parents and other tasks so in the trace there will be multiple tasks failed with the same or similar exception. It would be useful to know which task was the "root cause".