fix(pageserver): queue stopped error should be ignored during create timeline (#9767)

close https://github.com/neondatabase/neon/issues/9730

The test case tests if anything goes wrong during pageserver restart +
*during timeline creation not complete*. Therefore, queue is stopped
error is normal in this case, except that it should be categorized as a
shutdown error instead of a real error.

## Summary of changes

* More comments for the test case.
* Queue stopped error will now be forwarded as
CreateTimelineError::ShuttingDown.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

pageserver/src/tenant.rs

@@ -2446,6 +2446,12 @@ impl Tenant {
             .remote_client
             .wait_completion()
             .await
+            .map_err(|e| match e {
+                WaitCompletionError::NotInitialized(
+                    e, // If the queue is already stopped, it's a shutdown error.
+                ) if e.is_stopping() => CreateTimelineError::ShuttingDown,
+                e => CreateTimelineError::Other(e.into()),
+            })
             .context("wait for timeline initial uploads to complete")?;
 
         // The creating task is responsible for activating the timeline.