Identify source of the exception in a task trace

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.

The issue names no files, tests, or entry points. Start by locating ParSeq's exception propagation and task-trace implementation, then determine how a source task can be represented and verified. Done means a trace distinguishes the task that originally produced an exception from tasks that propagated it.

Written by the indexing model from the issue text.