pro-forma selection for improved error message;

--- a/src/Pure/Tools/ml_process.scala	Wed Mar 16 22:06:05 2016 +0100
+++ b/src/Pure/Tools/ml_process.scala	Wed Mar 16 22:16:58 2016 +0100
@@ -31,7 +31,8 @@
     val heaps: List[String] =
       if (raw_ml_system) Nil
       else {
-        val session_tree = tree.getOrElse(Sessions.load(options, dirs))
+        val (_, session_tree) =
+          tree.getOrElse(Sessions.load(options, dirs)).selection(sessions = List(logic_name))
         (session_tree.ancestors(logic_name) ::: List(logic_name)).
           map(a => File.platform_path(store.heap(a)))
       }