more robust: handle unavailable statistics;

--- a/src/Pure/ML/ml_statistics.ML	Wed Jul 15 16:28:26 2020 +0200
+++ b/src/Pure/ML/ml_statistics.ML	Wed Jul 15 17:10:26 2020 +0200
@@ -133,6 +133,7 @@
        TextIO.flushOut TextIO.stdOut;
        OS.Process.sleep (Time.fromReal delay);
        loop ());
-  in loop () handle Interrupt => OS.Process.exit OS.Process.success end;
+    fun exit () = OS.Process.exit OS.Process.success;
+  in loop () handle Interrupt => exit () | Fail _ => exit () end;
 
 end;