proper message text (see also fa62e095d8f1);

--- a/src/Pure/PIDE/prover.scala	Sat Mar 18 16:43:40 2017 +0100
+++ b/src/Pure/PIDE/prover.scala	Sat Mar 18 18:57:14 2017 +0100
@@ -68,7 +68,7 @@
   class Protocol_Output(props: Properties.T, val bytes: Bytes)
     extends Output(XML.Elem(Markup(Markup.PROTOCOL, props), Nil))
   {
-    lazy val text: String = bytes.toString
+    lazy val text: String = bytes.text
   }
 }