Thrasher: log backtrace of thrown exception

Thrasher: log backtrace of thrown exception

* add a wrapper to log uncaught exception to self.logger, greenlet also
  prints the backtrace and exception to stderr, but teuthology.log does
  not capture stderr. so we need to catch them by ourselves to reveal
  more info to root-cause this issue.
* log uncaught exception thrown by Thrasher.do_thrash() to self.log.

See: #10630
Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 64de3cd574b200d90cb3aea0d646a6236d8fec5e)