Bug 251463 – Error on piped ReadableStream leads to Unhandled Promise Rejection

• Status: RESOLVED
• Resolution: FIXED
• Component: JavaScriptCore

Hey, the following code leads to an Unhandled Promise Rejection on some websites: 

```
new ReadableStream({ start: c => c.error("error") }).pipeTo(new WritableStream()).catch(() => {})
```

It also happens when calling controller.error after the pipe:

```
new ReadableStream({ start: c => window.c = c }).pipeTo(new WritableStream()).catch(() => {})

c.error("error")
```

Just run it in Safari console on latest macOS (Ventura 13.2) and latest iOS (16.0)

Websites where it happens:
- example.com
- bugs.webkit.org
- github.com
- facebook.com
- amazon.com
- blankwebsite.com

Websites where it doesn't happen:
- google.com

[Radar WebKit Bug Importer]

<rdar://problem/105149496>

[youenn fablet]

Pull request: https://github.com/WebKit/WebKit/pull/11864

[EWS]

Committed 262137@main (3a75b5d2de94): <https://commits.webkit.org/262137@main>

Reviewed commits have been landed. Closing PR #11864 and removing active labels.