Handle errors from JSON serialization. (#666)

spec/index.bs

@@ -1208,6 +1208,7 @@ To <dfn>fetch an identity assertion</dfn> given a {{USVString}}
     1. If |provider|'s {{IdentityProviderRequestOptions/params}} is not empty:
         1. Let |json| be the result of [=serializing a JavaScript value to a JSON string=]
             with |provider|'s {{IdentityProviderRequestOptions/params}}.
+        1. If serializing threw an exception, return failure.
         1. Append ("params", |json|) to |list|.
     1. Let |requestBody| be the result of running [=urlencoded serializer=] with |list|.
     1. Let |request| be a new <a spec=fetch for=/>request</a> as follows: