When a PathFollow is badly configured it's possible to have code that
call get_progress_ratio just after set_progress_ratio does not return
the value just set, which may be confusing for developer (ie that
myPathFollow2D.set_progress_ratio(0.5)
myPathFollow2D.get_progress_ratio()
does not return 0.5)
This commit makes ensures the developer has useful error messages in
such case, to make it easier to troubleshot it.
ea9dff87ae