Since [37573], `get_object_term_cache()` has expected term IDs to be
stored in the taxonomy relationship cache. The function would then
reach directly into the 'terms' cache to fetch the data corresponding
to a given term, before returning a `WP_Term` object. This caused
problems when, for one reason or another, term data was cached
inconsistently:
* If the 'terms' cache is empty for a given term ID, despite the earlier call to `_prime_term_caches()`, `get_term()` would return an error object.
* If the array of cached term IDs contains an invalid ID, `get_term()` would return an error object.
We avoid these errors by no longer touching the 'terms' cache directly,
but running term IDs through `get_term()` and allowing that function to
reference the cache (and database, as needed). If `get_term()` returns
an error object for any of the cached term IDs, `get_object_term_cache()`
will return that error object alone. This change ensures that upstream
functions, like `get_the_terms()`, return `WP_Error` objects in a
predictable fashion.
Props dd32, michalzuber.
Fixes#37291.
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