`add_term_meta()` and `update_term_meta()` identify terms by `$term_id`. In
cases where a term is shared between taxonomies, `$term_id` is insufficient to
distinguish where the metadata belongs.
When attempting to add/update termmeta on a shared term, a `WP_Error` object
is returned. This gives developers enough information to decide whether they'd
like to force the term to be split and retry the save, or show an error in the
UI, or whatever.
Props boonebgorges, mboynes, DH-Shredder, jorbin, aaroncampbell.
Fixes#34544.
git-svn-id: https://develop.svn.wordpress.org/trunk@35515 602fd350-edb4-49c9-b593-d223f7449a82
7f79d26ca5