more robust;

--- a/src/Pure/System/progress.scala	Thu Aug 17 20:06:24 2023 +0200
+++ b/src/Pure/System/progress.scala	Thu Aug 17 20:13:49 2023 +0200
@@ -274,6 +274,10 @@
 extends Progress {
   database_progress =>
 
+  if (UUID.unapply(context_uuid).isEmpty) {
+    error("Bad Database_Progress.context_uuid: " + quote(context_uuid))
+  }
+
   private var _agent_uuid: String = ""
   private var _context: Long = -1
   private var _serial: Long = 0