Returns current total output written to standard error.
        :param tail: Return this number of most-recent lines.
        :return: copy of stderr stream

def get_stderr(self, tail=None):
        """
        Returns current total output written to standard error.
        :param tail: Return this number of most-recent lines.
        :return: copy of stderr stream
        """
        if self.finished(): 
            self.join_threads()
        while not self.stderr_q.empty():
            self.stderr_l.append(self.stderr_q.get_nowait())
        if tail is None:
            tail = len(self.stderr_l)
        return _py2_and_3_joiner('\n', self.stderr_l[:tail])