clarified interface;

--- a/src/Pure/System/progress.scala	Sun May 14 16:54:03 2017 +0200
+++ b/src/Pure/System/progress.scala	Sun May 14 17:00:57 2017 +0200
@@ -16,6 +16,9 @@
   def echo_if(cond: Boolean, msg: String) { if (cond) echo(msg) }
   def theory(session: String, theory: String) {}
 
+  def echo_warning(msg: String) { echo(Output.warning_text(msg)) }
+  def echo_error(msg: String) { echo(Output.error_text(msg)) }
+
   def stopped: Boolean = false
   override def toString: String = if (stopped) "Progress(stopped)" else "Progress"