Add structured logging and better error handling

internal/fetch/session.go

@@ -123,6 +123,8 @@ func (session *session) saveCheckpoint() {
 		return
 	}
 	state := session.snapshot()
-	session.file.Sync()
+	if err := session.file.Sync(); err != nil {
+		session.request.logger().Warn("could not flush partial file to disk", "error", err)
+	}
 	session.request.Checkpoint(state)
 }