Continue downloading card images after failing url on one.

I should probably bust this out to a method.

cockatrice/src/carddatabase.cpp

@@ -236,6 +236,8 @@ void PictureLoader::startNextPicDownload()
     QString picUrl = getPicUrl(cardBeingDownloaded.getCard());
     if (picUrl.isEmpty()) {
         qDebug() << "No url for" << cardBeingDownloaded.getCard()->getName();
+        cardBeingDownloaded = 0;
+        downloadRunning = false;
         return;
     }