qa/tasks: add exception in do_thrash()

qa/tasks: add exception in do_thrash()

Fixes the bug during DaemonWatchdog testing:

Traceback (most recent call last):
  File "/home/teuthworker/src/github.com_ceph_ceph-c_wip-daemonwatchdog-testing13/qa/tasks/daemonwatchdog.py", line 38, in _run
    self.watch()
  File "/home/teuthworker/src/github.com_ceph_ceph-c_wip-daemonwatchdog-testing13/qa/tasks/daemonwatchdog.py", line 112, in watch
    if thrasher.e is not None:
AttributeError: MonitorThrasher instance has no attribute 'e'

Signed-off-by: Jos Collin <jcollin@redhat.com>