src/Pure/Concurrent/isabelle_thread.scala

   override def interrupt: Unit = handler(thread)
 
   def interrupt_handler[A](new_handler: Isabelle_Thread.Interrupt_Handler)(body: => A): A =
     if (new_handler == null) body
     else {
-      require(is_self)
+      require(is_self, "interrupt handler on other thread")
 
       val old_handler = handler
       handler = new_handler
       try {
         if (clear_interrupt) interrupt