tuned message according to ML version;

--- a/src/Pure/Thy/sessions.scala	Sat Sep 29 14:58:01 2018 +0200
+++ b/src/Pure/Thy/sessions.scala	Sat Sep 29 17:08:07 2018 +0200
@@ -210,7 +210,7 @@
             space_explode(',', options.string("condition")).
               filter(cond => Isabelle_System.getenv(cond) == "")
           if (conditions.nonEmpty) {
-            warn("condition " + conditions.mkString(" "))
+            warn("undefined " + conditions.mkString(", "))
             false
           }
           else if (default_skip_proofs && !options.bool("skip_proofs")) {