I exited a locomotive with the Quick Ride hotkey on accident, which picked it up and dropped it on the ground without telling me (my inventory was full).
But I had locomotives disabled in the Quick Ride Vehicle GUI. I would expect Quick Ride-style exiting to not work for disabled vehicles, just like Quick Ride-style entering doesn't work.

Ideally, it would exit the vehicle normally leaving it there, but I think displaying an error message and doing no action would also be fine.

Sorry for the long delay. You make a good point, it would make sense for the mod to completely ignore locomotives if they are disabled, even for exiting. I'll probably start working on a small update soon and I'll try to implement that at the same time.