When a gathering thread raises an exception, it gets forwarded to the
main thread and re-raised there. However, during this transition it
loses details about exact location of the problem.
This patch creates an extended traceback in the worker, which should
make it easier to track the problem down later.
JIRA: RHELCMP-4259
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>