Now checking for errors before parsing newznab feeds

NzbDrone.Core/Indexers/Newznab/NewznabPreProcessor.cs

@@ -0,0 +1,24 @@
+using System;
+using System.Linq;
+using System.Xml.Linq;
+using NzbDrone.Core.Indexers.Exceptions;
+
+namespace NzbDrone.Core.Indexers.Newznab
+{
+    public static class NewznabPreProcessor
+    {
+        public static void Process(string source, string url)
+        {
+            var xdoc = XDocument.Parse(source);
+            var error = xdoc.Descendants("error").FirstOrDefault();
+
+            if (error == null) return;
+
+            var code = Convert.ToInt32(error.Attribute("code").Value);
+
+            if (code >= 100 && code <= 199) throw new ApiKeyException("Invalid API key: {0}");
+
+            throw new NewznabException("Newznab error detected: {0}", error.Attribute("description").Value);
+        }
+    }
+}