"argmax of empty sequence" in autofocus

When I run fast autofocus, if there is no stage connected it fails with

ValueError: attempt to get argmax of an empty sequence

Connecting the stage, then running ofm restart fixes the problem.

The solution to this is probably to:

1. Provide a way to check there is a stage connected
2. Use this at the start of the fast autofocus function, and raise a helpful error if the stage is not connected.