fix: catch unhandled promise rejection in WebTransport muxer

[achingbrain]

If the WebTransport session has closed, trying to read new stream can cause the promise to reject, printing an error in the console.

This is non-fatal for browsers but let's catch and log the error instead.

Change checklist

• [x] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation if necessary (this includes comments as well)
• [ ] I have added tests that prove my fix is effective or that my feature works