Don't try to download small version of non-existing poster

src/NzbDrone.Core/MetadataSource/TraktProxy.cs

@@ -187,7 +187,7 @@ namespace NzbDrone.Core.MetadataSource
 
         private static string GetPosterThumbnailUrl(string posterUrl)
         {
-            if (posterUrl.Contains("poster-small.jpg")) return posterUrl;
+            if (posterUrl.Contains("poster-small.jpg") || posterUrl.Contains("poster-dark.jpg")) return posterUrl;
 
             var extension = Path.GetExtension(posterUrl);
             var withoutExtension = posterUrl.Substring(0, posterUrl.Length - extension.Length);