`membrane_grant_single` would skip the grant if someone tried to grant
a sealer unsealer (since sealer/unsealers are not supposed to ever be
wrapped). However, `cn_membrane_grant` would still update the previous
owner to be the membrane's secret cspace despite the fact that the grant
never occurred so the sealer/unsealer did not exist in that c-space.

This commit adds a new error code ESKIPPED that can be returned to
signal that an operation was skipped, so tracking data structures don't
necessarily need to be updated.