output physical_stderr, e.g. for low-level debugging;

--- a/src/Pure/Tools/build.scala	Mon Aug 12 14:39:55 2019 +0200
+++ b/src/Pure/Tools/build.scala	Mon Aug 12 15:22:32 2019 +0200
@@ -289,6 +289,7 @@
             }
 
           process.result(
+            progress_stderr = Output.writeln(_),
             progress_stdout = (line: String) =>
               Library.try_unprefix("\floading_theory = ", line) match {
                 case Some(theory) =>