pageserver: error type for collect_keyspace (#5846)

## Problem

This is a log hygiene fix, for an occasional test failure.

warn-level logging in imitate_timeline_cached_layer_accesses can't
distinguish actual errors from shutdown cases.

## Summary of changes

Replaced anyhow::Error with an explicit CollectKeySpaceError type, that
includes conversion from PageReconstructError::Cancelled.

pageserver/src/http/routes.rs

@@ -1478,7 +1478,7 @@ async fn timeline_collect_keyspace(
         let keys = timeline
             .collect_keyspace(at_lsn, &ctx)
             .await
-            .map_err(ApiError::InternalServerError)?;
+            .map_err(|e| ApiError::InternalServerError(e.into()))?;
 
         json_response(StatusCode::OK, Partitioning { keys, at_lsn })
     }