Tweaked semantics of error prototype.

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index d6de768e5bed2f7b4947f613637f5f821e279b7e..c2fc5fc74de67f3fc3a0bd9243a5123bccf46cf9 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -634,6 +634,7 @@ function DefineError(f) {
     // it hasn't been properly configured yet.  Instead we create a
     // special not-a-true-error-but-close-enough object.
     function ErrorPrototype() {}
+    %FunctionSetPrototype(ErrorPrototype, $Object.prototype);
     %FunctionSetInstanceClassName(ErrorPrototype, 'Error');
     %FunctionSetPrototype(f, new ErrorPrototype());
   } else {