avoid markup, for the sake of Build_Log.Log_File.parse_props;

--- a/src/Pure/Tools/build.ML	Fri Jun 09 13:56:32 2017 +0200
+++ b/src/Pure/Tools/build.ML	Fri Jun 09 14:25:00 2017 +0200
@@ -221,7 +221,7 @@
     val _ = Options.load_default ();
     val _ = Isabelle_Process.init_options ();
     val args = decode_args (File.read (Path.explode args_file));
-    fun error_message msg = writeln ("\ferror_message = " ^ encode_lines msg);
+    fun error_message msg = writeln ("\ferror_message = " ^ encode_lines (YXML.content_of msg));
     val _ =
       Unsynchronized.setmp Private_Output.protocol_message_fn protocol_message
         build_session args