GH-1697 always stale files tolerate errors if a local copy is present

This fixes the situation when liteloader snapshot site is broken
and there's an older local snapshot already present.

api/logic/net/Sink.h

@@ -17,6 +17,7 @@ public: /* methods */
 	virtual JobStatus write(QByteArray & data) = 0;
 	virtual JobStatus abort() = 0;
 	virtual JobStatus finalize(QNetworkReply & reply) = 0;
+	virtual bool hasLocalData() = 0;
 
 	void addValidator(Validator * validator)
 	{