Fix MTU detection below 1280 with IPv6 on windows

[Serock3]

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This change is 

[dlon]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @Serock3)

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talpid-windows/src/net.rs line 339 at r1 (raw file):

    let mut row = match get_ip_interface_entry(ip_family, &luid) {
        Ok(row) => row,
        Err(error) if error.raw_os_error() == Some(ERROR_NOT_FOUND as i32) => return Ok(()),

I think we should return this error. It makes little sense to ignore the error given that the caller is specifically asking to set the MTU for the given family. Also, both interfaces/families should always exist when we call this.

[dlon]

Reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved