Fixed: Ensure indexer errors are handled before processing response

(cherry picked from commit 76f93c8415419f0c3dab90582d47a1c9a653263c)

Closes #2458

src/NzbDrone.Core/Indexers/Newznab/NewznabRssParser.cs

@@ -50,6 +50,11 @@ namespace NzbDrone.Core.Indexers.Newznab
 
         protected override bool PreProcess(IndexerResponse indexerResponse)
         {
+            if (indexerResponse.HttpResponse.HasHttpError)
+            {
+                base.PreProcess(indexerResponse);
+            }
+
             var xdoc = LoadXmlDocument(indexerResponse);
 
             CheckError(xdoc, indexerResponse);