http://www.yiibai.com/java/io/printwriter_checkerror.html
http://docs.oracle.com/javase/7/docs/api/java/io/PrintWriter.html#checkError()
public boolean checkError()
Flushes the stream if it's not closed and checks its error state.
Returns:
true if the print stream has encountered an error, either on the underlying output stream or during a format conversion.
如果流没有关闭，Flush(从缓存中输出数据并缓存)。然后检查流的错误状态
如果有错误才返回 true
这个应该跟 setError 和 clearError 有关