A problem was hit with pytest.fail with raises Failed exception not
being propagated to .wait. As it turned out it was not propagated
because pytest's Failed derives from BaseException, not Exception, and
we were catching only Exception and its children.

Rework the code to propagate all exception types from workers.

Performance change is with noise (it is either a bit faster for one set
of runs, or a bit slower for another set of runs).