I ended up pulling out the `instance?` check here as part of the work in a new commit, cbeddbd.  The implementation of the old `translate-app-shutdown-error-into-throwable` moved to `app->shutdown-error-throwable-or-app` in the *internal* namespace.  I had been thinking that since an `:error` should always derive from `Throwable` that it might be better to go ahead and let this blow up if `:error` were set to some non-`Throwable` type than to cover this up here.