more informative error (stderr);

--- a/src/Pure/Admin/isabelle_cronjob.scala	Tue Oct 25 12:36:09 2016 +0200
+++ b/src/Pure/Admin/isabelle_cronjob.scala	Tue Oct 25 14:06:43 2016 +0200
@@ -183,6 +183,7 @@
         res match {
           case Exn.Res(_) => None
           case Exn.Exn(exn) =>
+            exn.printStackTrace()
             val first_line = Library.split_lines(Exn.message(exn)).headOption getOrElse "exception"
             Some(first_line)
         }