Omit " and a user agent-defined message".

As suggested in the review, I have omitted the phrase.

docs/index.bs

@@ -2139,7 +2139,7 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
                 1. Invoke [=Batch Cache Operations=] with |operations|. If this [=throws=] an exception, set |errorData| to the exception.
                 1. [=Queue a task=], on |cacheJobPromise|'s [=relevant settings object=]'s [=responsible event loop=] using the [=DOM manipulation task source=], to perform the following substeps:
                     1. If |errorData| is null, resolve |cacheJobPromise| with undefined.
-                    1. Else, reject |cacheJobPromise| with a [=exception/create|new=] [=exception=] with |errorData| and a user agent-defined [=DOMException/message=], in |realm|.
+                    1. Else, reject |cacheJobPromise| with a [=exception/create|new=] [=exception=] with |errorData|, in |realm|.
     </section>
 
     <section algorithm="cache-delete">
@@ -2168,7 +2168,7 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
                 1. If |errorData| is null, then:
                     1. If |requestResponses| [=list/is not empty=], resolve |cacheJobPromise| with true.
                     1. Else, resolve |cacheJobPromise| with false.
-                1. Else, reject |cacheJobPromise| with a [=exception/create|new=] [=exception=] with |errorData| and a user agent-defined [=DOMException/message=], in |realm|.
+                1. Else, reject |cacheJobPromise| with a [=exception/create|new=] [=exception=] with |errorData|, in |realm|.
         1. Return |cacheJobPromise|.
     </section>
 
@@ -2640,10 +2640,10 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
       : Output
       :: none
 
-      1. If |job|'s [=job/client=] is not null, [=queue a task=], on |job|'s [=job/client=]'s [=responsible event loop=] using the [=DOM manipulation task source=], to reject |job|'s [=job/job promise=] with a [=exception/create|new=] [=exception=] with |errorData| and a user agent-defined [=DOMException/message=], in |job|'s [=job/client=]'s [=environment settings object/Realm=].
+      1. If |job|'s [=job/client=] is not null, [=queue a task=], on |job|'s [=job/client=]'s [=responsible event loop=] using the [=DOM manipulation task source=], to reject |job|'s [=job/job promise=] with a [=exception/create|new=] [=exception=] with |errorData|, in |job|'s [=job/client=]'s [=environment settings object/Realm=].
       1. For each |equivalentJob| in |job|'s [=list of equivalent jobs=]:
           1. If |equivalentJob|'s [=job/client=] is null, [=iteration/continue=].
-          1. [=Queue a task=], on |equivalentJob|'s [=job/client=]'s [=responsible event loop=] using the [=DOM manipulation task source=], to reject |equivalentJob|'s [=job/job promise=] with a [=exception/create|new=] [=exception=] with |errorData| and a user agent-defined [=DOMException/message=], in |equivalentJob|'s [=job/client=]'s [=environment settings object/Realm=].
+          1. [=Queue a task=], on |equivalentJob|'s [=job/client=]'s [=responsible event loop=] using the [=DOM manipulation task source=], to reject |equivalentJob|'s [=job/job promise=] with a [=exception/create|new=] [=exception=] with |errorData|, in |equivalentJob|'s [=job/client=]'s [=environment settings object/Realm=].
   </section>
 
   <section algorithm>