pageserver: refine shutdown handling in secondary download (#8052)

## Problem

Some code paths during secondary mode download are returning Ok() rather
than UpdateError::Cancelled. This is functionally okay, but it means
that the end of TenantDownloader::download has a sanity check that the
progress is 100% on success, and prints a "Correcting drift..." warning
if not. This warning can be emitted in a test, e.g.
https://neon-github-public-dev.s3.amazonaws.com/reports/pr-8049/9503642976/index.html#/testresult/fff1624ba6adae9e.

## Summary of changes

- In secondary download cancellation paths, use
Err(UpdateError::Cancelled) rather than Ok(), so that we drop out of the
download function and do not reach the progress sanity check.

pageserver/src/tenant/secondary/downloader.rs

@@ -513,7 +513,7 @@ impl<'a> TenantDownloader<'a> {
         // cover our access to local storage.
         let Ok(_guard) = self.secondary_state.gate.enter() else {
             // Shutting down
-            return Ok(());
+            return Err(UpdateError::Cancelled);
         };
 
         let tenant_shard_id = self.secondary_state.get_tenant_shard_id();
@@ -846,7 +846,7 @@ impl<'a> TenantDownloader<'a> {
         for layer in timeline.layers {
             if self.secondary_state.cancel.is_cancelled() {
                 tracing::debug!("Cancelled -- dropping out of layer loop");
-                return Ok(());
+                return Err(UpdateError::Cancelled);
             }
 
             // Existing on-disk layers: just update their access time.