[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

@@ -622,6 +622,7 @@ public class UpdateCenter extends AbstractModelObject implements Saveable {
          * @see DownloadJob
          */
         public File download(DownloadJob job, URL src) throws IOException {
+            try {
                 URLConnection con = connect(job,src);
                 int total = con.getContentLength();
                 CountingInputStream in = new CountingInputStream(con.getInputStream());
@@ -653,6 +654,9 @@ public class UpdateCenter extends AbstractModelObject implements Saveable {
                 }
 
                 return tmp;
+            } catch (IOException e) {
+                throw new IOException2("Failed to download from "+src,e);
+            }
         }
 
         /**