当page为null时，意味着非正常的响应状态，应该抛出异常，否则SpiderListener的onSuccess方法和onError方法都会执行

Author: zhangheng09

webmagic-core/src/main/java/us/codecraft/webmagic/Spider.java

@@ -408,9 +408,7 @@ public void test(String... urls) {
     protected void processRequest(Request request) {
         Page page = downloader.download(request, this);
         if (page == null) {
-            sleep(site.getRetrySleepTime());
-            onError(request);
-            return;
+            throw new RuntimeException("unaccpetable response status");
         }
         // for cycle retry
         if (page.isNeedCycleRetry()) {