The shutdown command returns an error if stepdown fails with any error code other than NotMaster, even if force:true is specified. This can fail with InterruptedDueToReplStateChange if there is a concurrent stepdown in progress, since it kills user operations that take certain locks. In contrast, signal-based shutdown ignores all stepdown errors and proceeds with the shutdown. It is expected that shutdown with force:true would always shut down the server, so we should make it behave like signal-based shutdown.