Fix bug in watcher

If “utils.RunParts” were to raise an exception, a log message was
written and the code continued to run. Due to the exception the
“results” variable would not be defined.

Also change the code to log a backtrace (getting an exception is rather
unlikely and having a backtrace is useful) and update one comment.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>