qa: log warning on scrub error

qa: log warning on scrub error

Instead of printing the (useless) traceback, just print a warning about
ignoring the failure. The traceback makes it harder to search for the
real problem in the teuthology log.

Fixes: https://tracker.ceph.com/issues/43718
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
(cherry picked from commit b7454e423620e829e7417cdfca1faf5cd91dec3f)

Conflicts:
	qa/tasks/mon_thrash.py
- whereas master has "self.manager.raw_cluster_cmd('mon', 'scrub')" in
  the try block, in nautilus it is only "self.manager.raw_cluster_cmd('scrub')"