net/http: export `errCannotRewind`

https://github.com/golang/go/blob/dee3efd9f879335cfb583752b0729cc57f61897e/src/net/http/transport.go#L628

I'm having to resort to string matching to identify this error which is not ideal
```go
strings.Contains(err.Error(), "net/http: cannot rewind body after connection loss")
```
I'd love to just be able to use errors.Is
```go
errors.Is(err, http.ErrCannotRewind)
```