If saving a thumbnail image fails, delete the resulting file.

OpenJDK throws "javax.imageio.IIOException: Invalid argument to native writeImage", because it
does not support transparency, and that results in zero-sized thumbnail files being created.

Mage.Client/src/main/java/org/mage/plugins/card/images/ImageCache.java

@@ -298,6 +298,7 @@ public class ImageCache {
             }
         } catch (IOException e) {
             log.error(e,e);
+            imageFile.delete();
         }
         return image;
     }