New: Add logging is release is rejected because no download URL is available

(cherry picked from commit a83ed3bcce4b0a5ed5279029eed1b2ccfb4e8b85)

src/NzbDrone.Core/Indexers/HttpIndexerBase.cs

@@ -265,6 +265,7 @@ namespace NzbDrone.Core.Indexers
         {
             if (release.DownloadUrl.IsNullOrWhiteSpace())
             {
+                _logger.Trace("Invalid Release: '{0}' from indexer: {1}. No Download URL provided.", release.Title, release.Indexer);
                 return false;
             }