Fix test-error introduced by #10635.

PR #10635 introduced rescue from `ArgumentError` thrown by `Base64.strict_decode64`.

This broke natural order of things for `StaleSessionCheck#stale_session_check!` which tried auto_loading a class based on `ArgumentError` message , and later retrying the `Marshal#load` of class, successfully allowing auto_loading.

  This PR tries to fix this behavior by forwarding `ArgumentError` 's not raised  by `Base64.strict_decode64` , as is, ahead to `StaleSessionCheck#stale_session_check!`