backend-headless: Don't try to finish frame if it was cancelled

When multiple outputs are present, a failure in one's repaint will
cause any repaints of all other outputs to be cancelled. If this
happens during the headless "frame" time, the assertion on
repaint_status in weston_output_finish_frame fails.

Do the same as the DRM backend does in its equivalent callback
drm_output_update_complete and don't finish the frame.

Signed-off-by: Ray Smith <rsmith@brightsign.biz>