Bifurcate the expected error message based on runtime mode (#2821)

## Motivation and Context
Makes a failing test `multi_region_access_points` pass in the
orchestrator

## Description
The orchestrator has already an `AwsAuthStage` counterpart implemented
(which is where the expected error for `multi_region_access_points`
comes from in the middleware mode).

This PR just updates the expected error message in the test for the
orchestrator to make it pass.

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Co-authored-by: Yuki Saito <awsaito@amazon.com>