Avoid callbacks to user code during error formatting in a couple of

src/mirror-debugger.js

Index: src/mirror-debugger.js
diff --git a/src/mirror-debugger.js b/src/mirror-debugger.js
index 80d385952b425c4033a50763279cf6d77f3491b1..416f88794dadf07bbd481ae0f2dc8d152974947d 100644
--- a/src/mirror-debugger.js
+++ b/src/mirror-debugger.js
@@ -1084,9 +1084,9 @@ ErrorMirror.prototype.toText = function() {
   // Use the same text representation as in messages.js.
   var text;
   try {
-    str = builtins.ToDetailString(this.value_);
+    str = %_CallFunction(this.value_, builtins.errorToString);
   } catch (e) {
-    str = '#<an Error>';
+    str = '#<Error>';
   }
   return str;
 }