close iterator (#8)

spec.html

@@ -33,9 +33,13 @@ <h1>Math.sumExact ( _items_ )</h1>
       1. Set _next_ to ? IteratorStepValue(_iteratorRecord_).
       1. If _next_ is not ~done~, then
         1. Set _count_ to _count_ + 1.
-        1. If _count_ ≥ 2<sup>53</sup>, throw a *RangeError* exception.
+        1. If _count_ ≥ 2<sup>53</sup>, then
+          1. Let _error_ be ThrowCompletion(a newly created *RangeError* object).
+          1. Return ? IteratorClose(_iteratorRecord_, _error_).
         1. NOTE: The above case is not expected to be reached in practice and is included only so that implementations may rely on inputs being "reasonably sized" without violating this specification.
-        1. If _next_ is not a number, throw a *TypeError* exception.
+        1. If _next_ is not a number, then
+          1. Let _error_ be ThrowCompletion(a newly created *TypeError* object).
+          1. Return ? IteratorClose(_iteratorRecord_, _error_).
         1. Let _n_ be _next_.
         1. If _state_ is not ~not-a-number~, then
           1. If _n_ is *NaN*, set _state_ to ~not-a-number~.