public function getIncrementalErrorOutput()
    {
        $this->readPipesForOutput(__FUNCTION__);

        $latest = stream_get_contents($this->stderr, -1, $this->incrementalErrorOutputOffset);
        $this->incrementalErrorOutputOffset = ftell($this->stderr);

        if (false === $latest) {
            return '';
        }

        return $latest;
    }