tuned message -- more readable JSON;

--- a/src/Pure/Tools/server.scala	Thu Mar 15 22:28:20 2018 +0100
+++ b/src/Pure/Tools/server.scala	Thu Mar 15 22:41:59 2018 +0100
@@ -479,7 +479,7 @@
     _sessions.change_result(sessions =>
       sessions.get(id) match {
         case Some(session) => (session, sessions - id)
-        case None => error("No session " + quote(id))
+        case None => error("No session " + Library.single_quote(id))
       })
   }