deal with WebTransport stream cancelations at the beginning of the stream

[marten-seemann]

When the peer cancels a stream before we have received the stream header, AND the packet carrying the stream header is lost, it won't be retransmitted.
When reading a reset from a stream, we therefore might not know that the stream was a WebTransport stream. We can however infer it from the error code used.

We must not return an error from the hijacker callback, otherwise quic-go will close the entire connection, if we're a client and receive a bidirectional stream: https://github.com/lucas-clemente/quic-go/blob/8185d1b4e07264aaf61fc1d3e993c513b1054285/http3/client.go#L153-L166