unix: implicitly signal write errors to libuv user

Fix an infinite loop in the example below when the stream encounters
an EPIPE/ECONNRESET/etc. error:

    // keep writing until we start buffering
    while (stream->write_queue_size == 0) {
      uv_write_t* req = make_write_req();
      uv_buf_t buf = uv_buf_init("PING", 4);
      uv_write(req, stream, &buf, 1, write_cb);
    }

uv_write() does not return an error code on write errors, the error is
only reported to the callback.

Before this commit, uv_write() left stream->write_queue_size untouched
on error, meaning the caller had no way to find out about that error
until the next tick of the event loop - which in the example above
leads to an infinite loop because that next tick is indefinitely
postponed.

This commit works around that at the cost of some added internal
complexity.

Fixes joyent/node#5516.