Fix error handling in NamedPipe::write

[blackbeam]

Hi. This is a part of tokio-rs/tokio#3388.

I found an issue with NamedPipe::write, namely it hangs in WouldBlock if other side is disconnected. The problem is in error handling:

1. Errors are stashed into the state (here and here);
2. And then ignored with WouldBlock without changing the state (here).

Another improvement introduced in this PR is better handling of immediately finished operations. For immediately failed operations this means, that the caller won't falsely receive Ok(usize). For immediately succeeded operations this means, that the caller could track the progress with better precision.

[carllerche]

At a skim, the behavior change looks correct to me. I will let @Thomasdezeeuw comment on any style feedback.

[Thomasdezeeuw]

The couple of unreachable! statement aren't great, but I don't see a better alternative. I'll merge this as it fixes a bug.