pageserver: handle shutdown cleanly in layer download API (#10598)

## Problem

This API is used in tests and occasionally for support. It cast all
errors to 500.

That can cause a failure on the log checks:
https://neon-github-public-dev.s3.amazonaws.com/reports/main/13056992876/index.html#suites/ad9c266207b45eafe19909d1020dd987/683a7031d877f3db/

## Summary of changes

- Avoid using generic anyhow::Error for layer downloads
- Map shutdown cases to 503 in http route

pageserver/src/http/routes.rs

@@ -1472,7 +1472,13 @@ async fn layer_download_handler(
     let downloaded = timeline
         .download_layer(&layer_name)
         .await
-        .map_err(ApiError::InternalServerError)?;
+        .map_err(|e| match e {
+            tenant::storage_layer::layer::DownloadError::TimelineShutdown
+            | tenant::storage_layer::layer::DownloadError::DownloadCancelled => {
+                ApiError::ShuttingDown
+            }
+            other => ApiError::InternalServerError(other.into()),
+        })?;
 
     match downloaded {
         Some(true) => json_response(StatusCode::OK, ()),