Safer Focuser loss, direct stop from UI, failure management and linear robustness

This MR adds a few protections against device loss when running an autofocus.

It prevents the autofocus procedure from restarting automatically and prevents the completion handler from returning to the initial position when clicking the stop button.

It manages the stop action when the solver is detecting sources as autofocus cannot be interrupted during that interval (stellarsolver limitation).

It adds capture failure management and properly completes the autofocus procedure when timing out or losing connection with a device.

It lets the Linear algorithm continue its progress when a particular focuser position causes no star to be detected, providing better robustness against out-of-focus frames and allowing larger step spans.