Have "Handle Fetch" return error when request's streaming body is unusable (#1563)

Part of whatwg/fetch#1144

Author: yutakahirano

docs/index.bs

@@ -3032,6 +3032,9 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
                   1. Wait until |e|'s [=FetchEvent/wait to respond flag=] is unset.
                   1. If |e|'s [=FetchEvent/respond-with error flag=] is set, set |handleFetchFailed| to true.
                   1. Else, set |response| to |e|'s [=FetchEvent/potential response=].
+              1. If |response| is null, |request|'s [=request/body=] is not null, and |request|'s [=request/body=]'s [=Body/source=] is null, then:
+                  1. If |request|'s [=request/body=] is [=Body/unusable=], set |handleFetchFailed| to true.
+                  1. Else, [=ReadableStream/cancel=] |request|'s [=request/body=] with undefined.
               1. If |e|'s <a>canceled flag</a> is set, set |eventCanceled| to true.
               1. If |fetchInstance| is [=fetch/terminated=], then [=queue a task=] to [=AbortSignal/signal abort=] on |requestObject|'s {{Request/signal}}.