I can't reproduce it. I receive RelayNotFound error

Example:

use nostr_sdk::prelude::*;

#[tokio::main]
async fn main() {
    let client = Client::new(Keys::generate());
    let result = client.add_relay("ws://localhost:3333").await;
    println!("{:?}", result);
    let result = client.connect_relay("ws://localhost:3333").await;
    println!("{:?}", result);
}

Running target/debug/examples/relay-test
Ok(true)
Ok(())

Example:

use nostr_sdk::prelude::*;

#[tokio::main]
async fn main() {
    let client = Client::new(Keys::generate());
    let result = client.add_relay("ws://localhost:3333").await;
    println!("{:?}", result);
    let result = client.connect_relay("ws://localhost:3333").await;
    println!("{:?}", result);
}

Running target/debug/examples/relay-test
Ok(true)
Ok(())

You are adding it with Client::add_relay so it can't return the RelayNotFound error.

How can I add it thus that I get an error if the connection fails?

How can I add it thus that I get an error if the connection fails?

Ah ok, now I've understood. It try to establish connection in another thread and automatically reconnect.

I can consider adding a try_connect method, that return an error if fail.