serial console error message on node that is not running is unhelpful

If an instance is not running for some reason and an attempt to get a serial console is made, we get the following.

$ ./solo serial turbolift
Entering VM console.
Use ^q to exit.
Press enter to continue.
Error: Anyhow(failed to create serial websocket stream

Caused by:
    HTTP error: 200 OK)

Which is not at all helpful.

This can be reproduced as follows with a simple one node topology.

use libfalcon::{cli::run, error::Error, unit::gb, Runner};

#[tokio::main]
async fn main() -> Result<(), Error> {
    let mut d = Runner::new("solo");

    // nodes
    let turbolift = d.node("turbolift", "helios-2.9", 8, gb(8));
    run(&mut d).await?;

    Ok(())
}

pfexec ./solo launch
# wait for topology to come up
pfexec pkill propolis-server
./solo serial turbolift

Start with the ./solo serial command and trace the failed serial websocket stream shown in the report. Reproduce it after stopping propolis-server, then make the error identify that the node is not running and verify the serial command reports that condition clearly.