blocked executation.

def block(self, warn_only=False):
        """blocked executation."""
        self._state = "not finished"
        if self._return_code is None:
            proc = subprocess.Popen(
                self.cmd, cwd=self.cwd, env=self.env,
                stdout=subprocess.PIPE,
                stderr=subprocess.PIPE
            )
            self._stdout, self._stderr = proc.communicate(timeout=self.timeout)
            self._return_code = proc.returncode

        self._state = FINISHED
        if not warn_only:
            self.raise_for_error()