src/Pure/General/exn.scala

--- a/src/Pure/General/exn.scala	Tue Mar 14 18:08:21 2017 +0100
+++ b/src/Pure/General/exn.scala	Tue Mar 14 19:40:39 2017 +0100
@@ -20,7 +20,7 @@
       }
     override def hashCode: Int = message.hashCode
 
-    override def toString: String = "ERROR(" + message + ")"
+    override def toString: String = "ERROR(" + Output.clean_yxml(message) + ")"
   }
 
   object ERROR