mononoke/fastreplay: add context on cache warmup failures

Summary: When it fails, it's better to know which repo failed.

Reviewed By: farnz

Differential Revision: D20245375

fbshipit-source-id: 9794911308dbdd67b20673857ac8b7b54f06a217

eden/mononoke/fastreplay/src/main.rs

@@ -334,7 +334,9 @@ async fn bootstrap_repositories<'a>(
 
             if let Some(warmup) = warmup {
                 info!(&logger, "Waiting for cache warmup to complete...");
-                warmup.await??;
+                warmup
+                    .await?
+                    .with_context(|| format!("Performing cache warmup on {}", name))?;
             }
 
             Result::<_, Error>::Ok((name, dispatcher))