Allow "get the next iteration result" to reject

Fixes #803.

Author: domenic

index.bs

@@ -4301,8 +4301,8 @@ Prose accompanying an [=interface=] with an [=asynchronously iterable declaratio
 <dfn id="dfn-get-the-next-iteration-result">get the next iteration result</dfn> algorithm.
 This algorithm receives the instance of the [=interface=] that is being iterated, as well as the
 async iterator itself (which can be useful for storing state).
-It must return a {{Promise}} that either resolves with undefined – to signal the end of the
-iteration – or a tuple with two elements:
+It must return a {{Promise}} that either rejects, resolves with undefined to signal the end of the
+iteration, or resolves with a tuple containing two elements:
 
 1.  a value of the first type given in the declaration;
 1.  a value of the second type given in the declaration.