Incorrect WP_Error usage in WP_Tax_Query

[SergeyBiryukov]

In WP_Tax_Query::clean_query() and WP_Tax_Query::transform_query(), strings are used as error codes:

$query = new WP_Error( 'Invalid taxonomy' );
...
$query = new WP_Error( 'Inexistent terms' );

Correct usage would be:

$query = new WP_Error( 'invalid_taxonomy', __( 'Invalid taxonomy.' ) );
...
$query = new WP_Error( 'inexistent_terms', __( 'Inexistent terms.' ) );

Introduced in [17686].

[SergeyBiryukov]

In 38079:

Taxonomy: Correct WP_Error usage in WP_Tax_Query::clean_query() and WP_Tax_Query::transform_query().

Fixes #37389.