rgw: deleting bucket can fail when it contains unfinished multipart uploads
As part of the process of deleting a bucket, all underway multipart uploads are aborted and their artifacts are cleaned up. This is done in blocks, and as long as the current block indicates that it is not the final block (i.e., is_truncated) it will fetch another block.
If the final block indicates it could not find the artifacts it wanted to clean up, the process fails and causes bucket deletion to fail. This does not happen if the final block succeeds and previous blocks have this error.
Since this should be a best-effort process, the inability to find artifacts should not prevent bucket deletion from completing.