Recovery fails if permissions holder has been deleted

Trying to recover a permission for someone how has now been delete results in a 400 response with:

{
    "domain": "global",
    "reason": "invalidSharingRequest",
    "message": "Bad Request. User message: \\"You are trying to invite ####@cam.ac.uk. Since there is no Google account associated with this email address, you must check the \\"Notify people\\" box to invite this recipient.\\""
}

Filtering to only active @cam.ac.uk may solve the problem in most cases but doesn't should a non-@cam.ac.uk address have the permission and has also been deleted.

Possibly better to have custom error handling that recognises the failure and skips the permission removal.

Alternatively, we perhaps should only restore @cam.ac.uk permissions.