more robust, notably wrt. interrupts;

--- a/src/HOL/Tools/Nitpick/kodkod.ML	Thu Mar 26 11:48:52 2020 +0100
+++ b/src/HOL/Tools/Nitpick/kodkod.ML	Sat Mar 28 22:18:28 2020 +0100
@@ -1079,6 +1079,7 @@
                 Normal (ps, js, first_error)
             end
         in remove_temporary_files (); outcome end
+        handle exn => (remove_temporary_files (); Exn.reraise exn)
       end
   end