Return an error when adding a term to a non-existent parent.

Parallels the logic of wp_insert_term(), introduced in [29196].

Props jesin.
Fixes #29614.
Built from https://develop.svn.wordpress.org/trunk@29867


git-svn-id: http://core.svn.wordpress.org/trunk@29627 1a063a9b-81f0-0310-95a4-ce76da25c4cd