pageserver: fix treating all download errors as 'Other' (#6836)

## Problem

`download_retry` correctly uses a fatal check to avoid retrying forever
on cancellations and NotFound cases. However, `download_layer_file` was
casting all download errors to "Other" in order to attach an
anyhow::Context.

Noticed this issue in the context of secondary downloads, where requests
to download layers that might not exist are issued intentionally, and
this resulted in lots of error spam from retries that shouldn't have
happened.

## Summary of changes

- Remove the `.context()` so that the original DownloadError is visible
to backoff::retry

pageserver/src/tenant/remote_timeline_client/download.rs

@@ -81,15 +81,7 @@ pub async fn download_layer_file<'a>(
                 .with_context(|| format!("create a destination file for layer '{temp_file_path}'"))
                 .map_err(DownloadError::Other)?;
 
-            let download = storage
-                .download(&remote_path, cancel)
-                .await
-                .with_context(|| {
-                    format!(
-                        "open a download stream for layer with remote storage path '{remote_path:?}'"
-                    )
-                })
-                .map_err(DownloadError::Other)?;
+            let download = storage.download(&remote_path, cancel).await?;
 
             let mut destination_file =
                 tokio::io::BufWriter::with_capacity(super::BUFFER_SIZE, destination_file);
@@ -98,9 +90,11 @@ pub async fn download_layer_file<'a>(
 
             let bytes_amount = tokio::io::copy_buf(&mut reader, &mut destination_file)
                 .await
-                .with_context(|| format!(
+                .with_context(|| {
+                    format!(
                     "download layer at remote path '{remote_path:?}' into file {temp_file_path:?}"
-                ))
+                )
+                })
                 .map_err(DownloadError::Other);
 
             match bytes_amount {