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.cpp

@@ -97,4 +97,9 @@ JobStatus FileSink::finalizeCache(QNetworkReply &)
 	return Job_Finished;
 }
 
+bool FileSink::hasLocalData()
+{
+	QFileInfo info(m_filename);
+	return info.exists() && info.size() != 0;
+}
 }