Source

File: wp-admin/includes/class-wp-ajax-upgrader-skin.php

public function error( $errors, ...$args ) {
		if ( is_string( $errors ) ) {
			$string = $errors;
			if ( ! empty( $this->upgrader->strings[ $string ] ) ) {
				$string = $this->upgrader->strings[ $string ];
			}

			if ( false !== strpos( $string, '%' ) ) {
				if ( ! empty( $args ) ) {
					$string = vsprintf( $string, $args );
				}
			}

			// Count existing errors to generate a unique error code.
			$errors_count = count( $this->errors->get_error_codes() );
			$this->errors->add( 'unknown_upgrade_error_' . ( $errors_count + 1 ), $string );
		} elseif ( is_wp_error( $errors ) ) {
			foreach ( $errors->get_error_codes() as $error_code ) {
				$this->errors->add( $error_code, $errors->get_error_message( $error_code ), $errors->get_error_data( $error_code ) );
			}
		}

		parent::error( $errors, ...$args );
	}