show user error as on command-line, e.g. relevant for unexpected crashes;

--- a/src/Pure/General/exn.scala	Tue Mar 14 19:46:53 2017 +0100
+++ b/src/Pure/General/exn.scala	Tue Mar 14 20:17:44 2017 +0100
@@ -20,7 +20,7 @@
       }
     override def hashCode: Int = message.hashCode
 
-    override def toString: String = "ERROR(" + Output.clean_yxml(message) + ")"
+    override def toString: String = Output.error_text(message)
   }
 
   object ERROR