Change tassert to allow ShutdownInProgress in MirrorMaestro's _mirror function

If mirroring a request fails in _mirror, we currently catch any exceptions and log them via a debug statement. There is a single tassert in _mirror which complains if the returned status of the response is ShutdownInProgress - this is unnecessary and is causing some tests to erroneously fail when mirroring requests race with shutdown. Don't trigger the tripwire if the status has the ShutdownInProgress error code.