Posts, Post Types: Fail gracefully when checking mapped cap against unregistered post status.

With `map_meta_cap` enabled for a post type, the `read_post` capability for posts with a public status is supposed to be mapped to the post type's `read` capability.

When a post is left in the database after the post status is no longer present, and WP does a `read_post` check against it, a PHP notice was thrown, and the cap check always failed.

As a more graceful fallback, the cap is now mapped onto `edit_others_posts`, which allows highly privileged users to be able to access orphaned content.

A `_doing_it_wrong()` notice is also added, so that developers and site administrators are aware that the cap mapping is failing in the absence of the registered post status.

Follow-up to [34091], which introduced a similar approach to checking mapped caps against an unregistered post type.

Props roytanck, SergeyBiryukov.
Fixes #48653.
Built from https://develop.svn.wordpress.org/trunk@47178


git-svn-id: https://core.svn.wordpress.org/trunk@46978 1a063a9b-81f0-0310-95a4-ce76da25c4cd