fix: use lazy argument evaluation to handle errors

An instance of `.then_some` on a bool should have been `.then`.
The former evaluates its arguments eagerly, so runs _regardless
of the value of the bool_, while the latter does so lazily, which
was the desired behavior.

In testing, all of our configurations had the file we were testing
the presence of before reading.  But other folks do not.

Fixes #9051

Author: dancrossnyc

sled-agent/src/bin/sled-agent.rs

@@ -62,7 +62,7 @@ async fn do_run() -> Result<(), CmdError> {
                 rss_config_path.push("config-rss.toml");
                 rss_config_path
             };
-            let rss_config = rss_config_path.exists().then_some({
+            let rss_config = rss_config_path.exists().then(|| {
                 let rss_config =
                     RackInitializeRequest::from_file(rss_config_path)
                         .map_err(|e| CmdError::Failure(anyhow!(e)))?;