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/FileSink.h

@@ -14,6 +14,7 @@ public: /* methods */
 	JobStatus write(QByteArray & data) override;
 	JobStatus abort() override;
 	JobStatus finalize(QNetworkReply & reply) override;
+	bool hasLocalData() override;
 
 protected: /* methods */
 	virtual JobStatus initCache(QNetworkRequest &);