improve message to notify user of ignoring this LDD

src/main/java/gov/nasa/pds/registry/common/util/file/FileDownloader.java

@@ -86,7 +86,7 @@ public void download(String fromUrl, File toFile) throws Exception
                 else
                 {
                   ignore.add(fromUrl);
-                  throw new Exception("Could not download " + fromUrl);
+                  throw new Exception("Could not download " + fromUrl + " and will not try again in this running instance.");
                 }
             }
         }