code: public void setTaskFailureException(Throwable taskFailureException) {
      if (!this.contains(ConfigurationKeys.TASK_FAILURE_EXCEPTION_KEY)) {
        this.setProp(ConfigurationKeys.TASK_FAILURE_EXCEPTION_KEY,
            Throwables.getStackTraceAsString(taskFailureException));
      }
    }
docs: If not already present, set the {@link ConfigurationKeys#TASK_FAILURE_EXCEPTION_KEY} to a {@link String}
  representation of the given {@link Throwable}.