process.stderr
process.stderr 属性会返回连接到 stderr (文件描述符 2) 的流。
它是一个 net.Socket（也就是 Duplex 流），除非文件描述符 2 指向文件（在这种情况下它是一个 Writable 流）。
process.stderr 与其他的 Node.js 流有重大区别。
详见进程 I/O 的注意事项。
The process.stderr property returns a stream connected to
stderr (fd 2). It is a net.Socket (which is a Duplex
stream) unless fd 2 refers to a file, in which case it is
a Writable stream.
process.stderr differs from other Node.js streams in important ways. See
note on process I/O for more information.