more robust: store important meta info before potential failure;

--- a/src/Pure/Admin/isabelle_devel.scala	Fri Jun 09 13:42:17 2017 +0200
+++ b/src/Pure/Admin/isabelle_devel.scala	Fri Jun 09 13:56:32 2017 +0200
@@ -72,6 +72,7 @@
     val store = Build_Log.store(options)
     using(store.open_database())(db =>
     {
+      store.update_database(db, log_dirs)
       store.update_database(db, log_dirs, ml_statistics = true)
       store.snapshot_database(db, root + Path.explode(BUILD_LOG_DB))
     })