Elevate "can't write hashes" message from debug to warning level

I never realized that my hashes weren't written, because it only
outputs a debug instead of a warning message.

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

core/qthelper.cpp

@@ -1108,7 +1108,7 @@ void write_hashes()
 		stream << thumbnailCache;
 		hashfile.commit();
 	} else {
-		qDebug() << "cannot open" << hashfile.fileName();
+		qWarning() << "Cannot open hashfile for writing: " << hashfile.fileName();
 	}
 }