[FIXED JENKINS-11598] improving the error diagnostics.

Note that the root cause isn't in Jenkins. See "Caused by: java.net.ConnectException: Connection refused: connect" line that indicates a problem in the network connectivity. But this change improves the error diagnostics.

core/src/main/java/hudson/model/UpdateCenter.java

@@ -622,37 +622,41 @@ public class UpdateCenter extends AbstractModelObject implements Saveable {
          * @see DownloadJob
          */
         public File download(DownloadJob job, URL src) throws IOException {
-            URLConnection con = connect(job,src);
-            int total = con.getContentLength();
-            CountingInputStream in = new CountingInputStream(con.getInputStream());
-            byte[] buf = new byte[8192];
-            int len;
+            try {
+                URLConnection con = connect(job,src);
+                int total = con.getContentLength();
+                CountingInputStream in = new CountingInputStream(con.getInputStream());
+                byte[] buf = new byte[8192];
+                int len;
 
-            File dst = job.getDestination();
-            File tmp = new File(dst.getPath()+".tmp");
-            OutputStream out = new FileOutputStream(tmp);
+                File dst = job.getDestination();
+                File tmp = new File(dst.getPath()+".tmp");
+                OutputStream out = new FileOutputStream(tmp);
 
-            LOGGER.info("Downloading "+job.getName());
-            try {
-                while((len=in.read(buf))>=0) {
-                    out.write(buf,0,len);
-                    job.status = job.new Installing(total==-1 ? -1 : in.getCount()*100/total);
+                LOGGER.info("Downloading "+job.getName());
+                try {
+                    while((len=in.read(buf))>=0) {
+                        out.write(buf,0,len);
+                        job.status = job.new Installing(total==-1 ? -1 : in.getCount()*100/total);
+                    }
+                } catch (IOException e) {
+                    throw new IOException2("Failed to load "+src+" to "+tmp,e);
                 }
-            } catch (IOException e) {
-                throw new IOException2("Failed to load "+src+" to "+tmp,e);
-            }
 
-            in.close();
-            out.close();
+                in.close();
+                out.close();
 
-            if (total!=-1 && total!=tmp.length()) {
-                // don't know exactly how this happens, but report like
-                // http://www.ashlux.com/wordpress/2009/08/14/hudson-and-the-sonar-plugin-fail-maveninstallation-nosuchmethoderror/
-                // indicates that this kind of inconsistency can happen. So let's be defensive
-                throw new IOException("Inconsistent file length: expected "+total+" but only got "+tmp.length());
-            }
+                if (total!=-1 && total!=tmp.length()) {
+                    // don't know exactly how this happens, but report like
+                    // http://www.ashlux.com/wordpress/2009/08/14/hudson-and-the-sonar-plugin-fail-maveninstallation-nosuchmethoderror/
+                    // indicates that this kind of inconsistency can happen. So let's be defensive
+                    throw new IOException("Inconsistent file length: expected "+total+" but only got "+tmp.length());
+                }
 
-            return tmp;
+                return tmp;
+            } catch (IOException e) {
+                throw new IOException2("Failed to download from "+src,e);
+            }
         }
 
         /**