Debug: Error reporting for SHashedImage loading

Overwrite QImage::load() in SHashedImage so that we can perform better
error reporting.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/imagedownloader.h

@@ -20,6 +20,7 @@ private:
 };
 
 class SHashedImage : public QImage {
+	bool load(const QString &fileName, const char *format=nullptr);
 public:
 	SHashedImage(struct picture *picture);
 };