color: define image description for gone output

These corner cases were found during the Weston implementation.

The compositor may have already destroyed an output, making its image description unavailable, while clients still hold on to the wl_output and/or wp_color_management_output_v1 objects. The request get_image_description must still work somehow, because a client cannot guarantee to avoid this race.

The simplest solution is to gracefully fail the image description.

If the client asks for an image description of an object it has made inert itself, raise a protocol error.