Fix when libgdiplus isn't present.

src/NzbDrone.Core/MediaCover/CoverAlreadyExistsSpecification.cs

@@ -46,11 +46,18 @@ namespace NzbDrone.Core.MediaCover
         {
             try
             {
+                GdiPlusInterop.CheckGdiPlus();
+
                 using (var bmp = new Bitmap(filename))
                 {
                 }
                 return true;
             }
+            catch (DllNotFoundException ex)
+            {
+                _logger.Error(ex, "Could not find libgdiplus. Cannot test if image is corrupt.");
+                return true;
+            }
             catch (Exception ex)
             {
                 _logger.Debug(ex, "Corrupted image found at: {0}. Redownloading...", filename);