The printer replies with state="failed" and data as a 2-element list
(["multi_color_box", [...]]) instead of the usual dict when it rejects
a manual ACE slot filament assignment (e.g. custom-RFID/third-party
material). _on_multicolor_box called data.get(...) unconditionally,
crashing with AttributeError and silently dropping the report -
including the regular slot-state update that would otherwise follow.
Add a state="failed" guard plus a defensive isinstance check, and
surface the rejection via _state["last_ams_set_error"] so the caller
of handle_api_ams_set_slot's optimistic cache update isn't left
showing a false success.
Note: this fixes the crash, not necessarily the underlying rejection
itself - the printer/ACE may still refuse assignments for material
types it doesn't recognize.
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