LibWeb: Remove exceptional return types from infallible stream IDL

Author: https://github.com/trflynn89
Commit: 2d4d16ac37
Pull-request: https://github.com/SerenityOS/serenity/pull/24165
Reviewed-by: https://github.com/kennethmyhra ✅
Reviewed-by: https://github.com/shannonbooth ✅

Userland/Libraries/LibWeb/Streams/ReadableStreamGenericReader.h

@@ -19,9 +19,9 @@ class ReadableStreamGenericReaderMixin {
 public:
     virtual ~ReadableStreamGenericReaderMixin() = default;
 
-    WebIDL::ExceptionOr<JS::GCPtr<JS::Promise>> closed();
+    JS::GCPtr<JS::Promise> closed();
 
-    WebIDL::ExceptionOr<JS::NonnullGCPtr<JS::Promise>> cancel(JS::Value reason);
+    JS::NonnullGCPtr<JS::Promise> cancel(JS::Value reason);
 
     JS::GCPtr<ReadableStream> stream() const { return m_stream; }
     void set_stream(JS::GCPtr<ReadableStream> stream) { m_stream = stream; }