more uniform and Java-conformant: change of handler is non-blocking and interrupts should not be exposed prematurely (reverting 220d19f3e074);

--- a/src/Pure/Concurrent/isabelle_thread.scala	Mon Apr 06 14:03:58 2020 +0200
+++ b/src/Pure/Concurrent/isabelle_thread.scala	Mon Apr 06 19:33:29 2020 +0200
@@ -162,7 +162,6 @@
     finally {
       handler = old_handler
       if (clear_interrupt) interrupt
-      Exn.Interrupt.expose()
     }
   }
 }