BaseView.remove_overlay() would crash with AttributeError if no overlay
was found. We now add a not_found_ok parameter (defaulting to True) that
makes the function silently return if the overlay could not be found.
Passing not_found_ok=False and catching OverlayNotFoundError can be
helpful in some scenarios to do something different if no overlay was
found.
Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>