proper result type (cf. b9f5cd845616);

--- a/src/Tools/VSCode/src/server.scala	Mon Sep 18 18:19:06 2017 +0200
+++ b/src/Tools/VSCode/src/server.scala	Mon Sep 18 18:25:52 2017 +0200
@@ -326,8 +326,8 @@
         delay_caret_update.revoke()
         delay_preview.revoke()
 
-        val rc = session.stop()
-        if (rc == 0) reply("") else reply("Prover shutdown failed: return code " + rc)
+        val result = session.stop()
+        if (result.ok) reply("") else reply("Prover shutdown failed: return code " + result.rc)
         None
       case None =>
         reply("Prover inactive")