Rotating sample changer: Stuck in a moving state [Timebox: 2d]

As a SANDALS scientist using the GEM sample changer I would like my sample changer not to get stuck in a moving state. Especially as in this state I can't issue new setpoints on the OPI. The issue may be related to some samples being down while others were not.

## Acceptance Criteria
* The issue is reproduced on the device and put into the emulator
* The issue is resolved and a test exists for it

Start by reproducing the GEM sample changer's stuck moving state on the device, especially when some samples are down and others are not. Then trace the corresponding emulator behavior and add a regression test; done means the device no longer blocks new OPI setpoints and the acceptance criteria are met.

Scritto dal modello di indicizzazione a partire dal testo della issue.