more robust message;

--- a/src/Pure/Admin/isabelle_cronjob.scala	Mon Jan 29 21:18:11 2024 +0100
+++ b/src/Pure/Admin/isabelle_cronjob.scala	Tue Jan 30 12:48:27 2024 +0100
@@ -476,8 +476,7 @@
         res match {
           case Exn.Res(_) => None
           case Exn.Exn(exn) =>
-            Output.writeln("Exception trace for " + quote(task.name) + ":")
-            exn.printStackTrace()
+            Output.writeln("Exception trace for " + quote(task.name) + ":\n" + Exn.trace(exn))
             val first_line = split_lines(Exn.message(exn)).headOption getOrElse "exception"
             Some(first_line)
         }