BibSched: show traceback when a task fails
BibSched: show traceback when a task fails

* Displays on stderr (and hence on the taske .err log file)
  a small traceback of the exception that caused the task to
  fail.

inveniosoftware/invenio,tiborsimko/invenio,tiborsimko/invenio,inveniosoftware/invenio