Extend exception message from Vec<T>::at
diff --git a/include/cxx.h b/include/cxx.h
index d3f6238..90c0524 100644
--- a/include/cxx.h
+++ b/include/cxx.h
@@ -481,8 +481,9 @@
 
 template <typename T>
 const T &Vec<T>::at(size_t n) const {
-  if (n >= this->size())
-    panic<std::out_of_range>("Vec");
+  if (n >= this->size()) {
+    panic<std::out_of_range>("rust::Vec index out of range");
+  }
   return (*this)[n];
 }